[#462] Implement PATCH for regular objects

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/handler/response.go

@@ -195,6 +195,15 @@ type PostResponse struct {
 	ETag   string `xml:"Etag"`
 }
 
+type PatchObjectResult struct {
+	Object PatchObject `xml:"Object"`
+}
+
+type PatchObject struct {
+	LastModified string `xml:"LastModified"`
+	ETag         string `xml:"ETag"`
+}
+
 // MarshalXML -- StringMap marshals into XML.
 func (s StringMap) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 	tokens := []xml.Token{start}