Do not require "charset=utf-8" for a schema1 with content type application/json

For compatibility with other registries that don't use this exact
variant of the Content-Type header, we need to be more flexible about
what we accept. Any form of "application/json" should be allowed. The
charset should not be included in the comparison.

See docker/docker#19400.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

manifests.go

@@ -2,6 +2,7 @@ package distribution
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/docker/distribution/context"
 	"github.com/docker/distribution/digest"
@@ -80,7 +81,17 @@ var mappings = make(map[string]UnmarshalFunc, 0)
 
 // UnmarshalManifest looks up manifest unmarshall functions based on
 // MediaType
-func UnmarshalManifest(mediatype string, p []byte) (Manifest, Descriptor, error) {
+func UnmarshalManifest(ctHeader string, p []byte) (Manifest, Descriptor, error) {
+	// Need to look up by the actual content type, not the raw contents of
+	// the header. Strip semicolons and anything following them.
+	var mediatype string
+	semicolonIndex := strings.Index(ctHeader, ";")
+	if semicolonIndex != -1 {
+		mediatype = ctHeader[:semicolonIndex]
+	} else {
+		mediatype = ctHeader
+	}
+
 	unmarshalFunc, ok := mappings[mediatype]
 	if !ok {
 		return nil, Descriptor{}, fmt.Errorf("unsupported manifest mediatype: %s", mediatype)