Fix ignored tag directives on unnamed fields

[klauspost]

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.