Decode non-JSON HTTP responses when possible

This doesn't affect anything in nilmdb, but is needed by nilmrun.

nilmdb/client/httpclient.py

@@ -109,7 +109,7 @@ class HTTPClient():
                                           stream=False, headers=headers)
         if isjson:
             return json.loads(response.content)
-        return response.content
+        return response.text
 
     def get(self, url, params=None):
         """Simple GET (parameters in URL)"""