Fix issue with del_worksheet

Make sure the response from the DELETE request when deleting a worksheet gets read by the client else we get a httplib.ResponseNotReady error on the next request to that HTTP connection.  Also added a test and a couple of entries in the test config.  Fixes #84.

Note that I am getting a couple of errors when running the unit tests that are unrelated to this change.  Perhaps I've not configured my test environment correctly.