Replace negotiation with a new method that can extract info

Alter how runtime.SerializeInfo is represented to simplify negotiation
and reduce the need to allocate during negotiation. Simplify the dynamic
client's logic around negotiating type. Add more tests for media type
handling where necessary.

pkg/runtime/codec.go

@@ -217,6 +217,22 @@ func (s base64Serializer) Decode(data []byte, defaults *unversioned.GroupVersion
 	return s.Serializer.Decode(out[:n], defaults, into)
 }
 
+// SerializerInfoForMediaType returns the first info in types that has a matching media type (which cannot
+// include media-type parameters), or the first info with an empty media type, or false if no type matches.
+func SerializerInfoForMediaType(types []SerializerInfo, mediaType string) (SerializerInfo, bool) {
+	for _, info := range types {
+		if info.MediaType == mediaType {
+			return info, true
+		}
+	}
+	for _, info := range types {
+		if len(info.MediaType) == 0 {
+			return info, true
+		}
+	}
+	return SerializerInfo{}, false
+}
+
 var (
 	// InternalGroupVersioner will always prefer the internal version for a given group version kind.
 	InternalGroupVersioner GroupVersioner = internalGroupVersioner{}