add vendor extensions to marshal openapi v2 header

Discovered while fuzzing a type converter to gnostic. Seems like it was forgotten to include this when originally implemented.

Author: Alexander Zielenski

pkg/validation/spec/header.go

@@ -53,7 +53,11 @@ func (h Header) MarshalJSON() ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	return swag.ConcatJSON(b1, b2, b3), nil
+	b4, err := json.Marshal(h.VendorExtensible)
+	if err != nil {
+		return nil, err
+	}
+	return swag.ConcatJSON(b1, b2, b3, b4), nil
 }
 
 // UnmarshalJSON unmarshals this header from JSON

pkg/validation/spec/header_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func float64Ptr(f float64) *float64 {
@@ -88,3 +89,19 @@ func TestIntegrationHeader(t *testing.T) {
 
 	assertParsesJSON(t, headerJSON, header)
 }
+
+// Makes sure that a Header unmarshaled from known good JSON, and one unmarshaled
+// from generated JSON are equivalent.
+func TestHeaderSerialization(t *testing.T) {
+	generatedJSON, err := json.Marshal(header)
+	require.NoError(t, err)
+
+
+	generatedJSONActual := Header{}
+	require.NoError(t, json.Unmarshal(generatedJSON, &generatedJSONActual))
+	require.EqualValues(t, header, generatedJSONActual)
+
+	goodJSONActual := Header{}
+	require.NoError(t, json.Unmarshal([]byte(headerJSON), &goodJSONActual))
+	require.EqualValues(t, header, goodJSONActual)
+}