added Unwrap method to errWrapped plus tests; switched travis to go 1.14

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 go:
-  - 1.12.x
+  - 1.14.x
   - tip
 
 env:

msgp/errors.go

@@ -123,6 +123,9 @@ func (e errWrapped) Resumable() bool {
 	return resumableDefault
 }
 
+// Unwrap returns the cause.
+func (e errWrapped) Unwrap() error { return e.cause }
+
 type errShort struct{}
 
 func (e errShort) Error() string   { return "msgp: too few bytes left to read object" }

msgp/errors_test.go

@@ -3,6 +3,7 @@ package msgp
 import (
 	"errors"
 	"fmt"
+	"io"
 	"strings"
 	"testing"
 )
@@ -87,3 +88,44 @@ func TestCauseShortByte(t *testing.T) {
 		t.Fatal()
 	}
 }
+
+func TestUnwrap(t *testing.T) {
+
+	// check errors that get wrapped
+	for idx, err := range []error{
+		errors.New("test"),
+		io.EOF,
+	} {
+		t.Run(fmt.Sprintf("wrapped_%d", idx), func(t *testing.T) {
+			cerr := WrapError(err, "test")
+			if cerr == err {
+				t.Fatal()
+			}
+			uwerr := errors.Unwrap(cerr)
+			if uwerr != err {
+				t.Fatal()
+			}
+			if !errors.Is(cerr, err) {
+				t.Fatal()
+			}
+		})
+	}
+
+	// check errors where only context is applied
+	for idx, err := range []error{
+		ArrayError{},
+		&ErrUnsupportedType{},
+	} {
+		t.Run(fmt.Sprintf("ctx_only_%d", idx), func(t *testing.T) {
+			cerr := WrapError(err, "test")
+			if cerr == err {
+				t.Fatal()
+			}
+			if errors.Unwrap(cerr) != nil {
+				t.Fatal()
+			}
+
+		})
+	}
+
+}