Fix NuxeoResponseCache in java client

While testing the last java client version, random error results have been encountered during a following usage on some documents (35):

• Checking if a document exists in the repository (via fetch "path" endpoint)
• Creating or Updating the document (via post/put "path" endpoints and same parent during all time)
• Attach blob to each document via "automation"

The cache is registering every endpoint calls of all methods (POST PUT DELETE GET).

• Error Results: some documents are not created at all (only 1, 2 or few)
• Expected result: having all of them created (all 35)

The method org.nuxeo.client.NuxeoClient#fetchResponse(retrofit2.Call<T>) is called in most of the endpoints provided in the client and not only the GET method ones -> IMO only GET commands should be cached.