Use constant to build correct url for service request

Author: spimminger

backmeup-storage-client/src/main/java/org/backmeup/storage/client/BackmeupStorageClient.java

@@ -49,7 +49,7 @@ public Metadata saveFile(String accessToken, String targetPath, boolean overwrit
 
         StringBuilder sb = new StringBuilder();
         sb.append(serviceUrl);
-        sb.append("FILE_RESOURCE");
+        sb.append(FILE_RESOURCE);
         sb.append(targetPath);
         if(overwrite){
             sb.append("?overwrite=");