api: Implement support for additional request headers.

Now GetObject and HeadObject both support

  - If-Modified-Since, If-Unmodified-Since
  - If-Match, If-None-Match

request headers.

These headers are used to further handle the responses for GetObject
and HeadObject API.

Fixes #1098

api-headers.go

@@ -76,7 +76,7 @@ func setObjectHeaders(w http.ResponseWriter, metadata fs.ObjectMetadata, content
 		setCommonHeaders(w)
 	}
 	// set object headers
-	lastModified := metadata.LastModified.Format(http.TimeFormat)
+	lastModified := metadata.LastModified.UTC().Format(http.TimeFormat)
 	// object related headers
 	w.Header().Set("Content-Type", metadata.ContentType)
 	if metadata.MD5 != "" {