[#131] Fix metadata in HeadObject and GetObject

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

api/handler/get.go

@@ -69,7 +69,7 @@ func writeHeaders(h http.Header, info *layer.ObjectInfo) {
 	h.Set(api.ETag, info.HashSum)
 
 	for key, val := range info.Headers {
-		h.Set("X-"+key, val)
+		h.Set(api.MetadataPrefix+key, val)
 	}
 }