Fix ignored tag directives on unnamed fields

Example:

```
type S struct {
	ABool       bool       `msg:",omitempty"`
}
```

... would ignore the omitempty, since all tags were replaced, resulting in output like:

```
// MarshalMsg implements msgp.Marshaler
func (z S) MarshalMsg(b []byte) (o []byte, err error) {
	o = msgp.Require(b, z.Msgsize())
	// map header, size 1
	// string "ABool"
	o = append(o, 0x81, 0xa5, 0x41, 0x42, 0x6f, 0x6f, 0x6c)
	o = msgp.AppendBool(o, z.ABool)
	return
}

```

Keep remaining tags when filling out the field name.

Code after:

```
// MarshalMsg implements msgp.Marshaler
func (z S) MarshalMsg(b []byte) (o []byte, err error) {
	o = msgp.Require(b, z.Msgsize())
	// check for omitted fields
	zb0001Len := uint32(1)
	var zb0001Mask uint8 /* 1 bits */
	_ = zb0001Mask
	if z.ABool == false {
		zb0001Len--
		zb0001Mask |= 0x1
	}
	// variable map header, size zb0001Len
	o = append(o, 0x80|uint8(zb0001Len))
	if zb0001Len == 0 {
		return
	}
	if (zb0001Mask & 0x1) == 0 { // if not omitted
		// string "ABool"
		o = append(o, 0xa5, 0x41, 0x42, 0x6f, 0x6f, 0x6c)
		o = msgp.AppendBool(o, z.ABool)
	}
	return
}
```

Regression test added.

_generated/omitempty.go

@@ -177,3 +177,50 @@ type NotOmitEmpty10 struct {
 	Field08 string `msg:"field08"`
 	Field09 string `msg:"field09"`
 }
+
+type OmitEmptyNoName struct {
+	ABool       bool       `msg:",omitempty"`
+	AInt        int        `msg:",omitempty"`
+	AInt8       int8       `msg:",omitempty"`
+	AInt16      int16      `msg:",omitempty"`
+	AInt32      int32      `msg:",omitempty"`
+	AInt64      int64      `msg:",omitempty"`
+	AUint       uint       `msg:",omitempty"`
+	AUint8      uint8      `msg:",omitempty"`
+	AUint16     uint16     `msg:",omitempty"`
+	AUint32     uint32     `msg:",omitempty"`
+	AUint64     uint64     `msg:",omitempty"`
+	AFloat32    float32    `msg:",omitempty"`
+	AFloat64    float64    `msg:",omitempty"`
+	AComplex64  complex64  `msg:",omitempty"`
+	AComplex128 complex128 `msg:",omitempty"`
+
+	ANamedBool    bool    `msg:",omitempty"`
+	ANamedInt     int     `msg:",omitempty"`
+	ANamedFloat64 float64 `msg:",omitempty"`
+
+	AMapStrStr map[string]string `msg:",omitempty"`
+
+	APtrNamedStr *NamedString `msg:",omitempty"`
+
+	AString    string `msg:",omitempty"`
+	AByteSlice []byte `msg:",omitempty"`
+
+	ASliceString      []string      `msg:",omitempty"`
+	ASliceNamedString []NamedString `msg:",omitempty"`
+
+	ANamedStruct    NamedStruct  `msg:",omitempty"`
+	APtrNamedStruct *NamedStruct `msg:",omitempty"`
+
+	AUnnamedStruct struct {
+		A string `msg:",omitempty"`
+	} `msg:",omitempty"` // omitempty not supported on unnamed struct
+
+	EmbeddableStruct `msg:",flatten,omitempty"` // embed flat
+
+	EmbeddableStruct2 `msg:",omitempty"` // embed non-flat
+
+	AArrayInt [5]int `msg:",omitempty"` // not supported
+
+	ATime time.Time `msg:",omitempty"`
+}

_generated/omitempty_test.go

@@ -93,6 +93,71 @@ func TestOmitEmpty0(t *testing.T) {
 	}
 }
 
+func TestOmitEmptyNoNames(t *testing.T) {
+	var s string
+
+	var oe0a OmitEmptyNoName
+
+	s = mustEncodeToJSON(&oe0a)
+	if s != `{"AUnnamedStruct":{},"AArrayInt":[0,0,0,0,0]}` {
+		t.Errorf("wrong result: %s", s)
+	}
+
+	var oe0b OmitEmptyNoName
+	oe0b.AString = "teststr"
+	s = mustEncodeToJSON(&oe0b)
+	if s != `{"AString":"teststr","AUnnamedStruct":{},"AArrayInt":[0,0,0,0,0]}` {
+		t.Errorf("wrong result: %s", s)
+	}
+
+	// more than 15 fields filled in
+	var oe0c OmitEmptyNoName
+	oe0c.ABool = true
+	oe0c.AInt = 1
+	oe0c.AInt8 = 1
+	oe0c.AInt16 = 1
+	oe0c.AInt32 = 1
+	oe0c.AInt64 = 1
+	oe0c.AUint = 1
+	oe0c.AUint8 = 1
+	oe0c.AUint16 = 1
+	oe0c.AUint32 = 1
+	oe0c.AUint64 = 1
+	oe0c.AFloat32 = 1
+	oe0c.AFloat64 = 1
+	oe0c.AComplex64 = complex(1, 1)
+	oe0c.AComplex128 = complex(1, 1)
+	oe0c.AString = "test"
+	oe0c.ANamedBool = true
+	oe0c.ANamedInt = 1
+	oe0c.ANamedFloat64 = 1
+
+	var buf bytes.Buffer
+	en := msgp.NewWriter(&buf)
+	err := oe0c.EncodeMsg(en)
+	if err != nil {
+		t.Fatal(err)
+	}
+	en.Flush()
+	de := msgp.NewReader(&buf)
+	var oe0d OmitEmptyNoName
+	err = oe0d.DecodeMsg(de)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// spot check some fields
+	if oe0c.AFloat32 != oe0d.AFloat32 {
+		t.Fail()
+	}
+	if oe0c.ANamedBool != oe0d.ANamedBool {
+		t.Fail()
+	}
+	if oe0c.AInt64 != oe0d.AInt64 {
+		t.Fail()
+	}
+}
+
 // TestOmitEmptyHalfFull tests mixed omitempty and not
 func TestOmitEmptyHalfFull(t *testing.T) {
 	var s string

parse/getast.go

@@ -413,7 +413,11 @@ func (fs *FileSet) getField(f *ast.Field) []gen.StructField {
 	sf[0].FieldElem = ex
 	if sf[0].FieldTag == "" {
 		sf[0].FieldTag = sf[0].FieldName
-		sf[0].FieldTagParts = []string{sf[0].FieldName}
+		if len(sf[0].FieldTagParts) <= 1 {
+			sf[0].FieldTagParts = []string{sf[0].FieldTag}
+		} else {
+			sf[0].FieldTagParts = append([]string{sf[0].FieldName}, sf[0].FieldTagParts[1:]...)
+		}
 	}
 
 	// validate extension