Check headers in response before Strings and JSON

This helps avoid a situation where:

* no content-type header is returned
* but json is expected by a response_json_paths
* and no json_data is created because that will only happen if there
  is a correct content-type

This will at least make sure that the initial error is the one
reported, rather than something caused by that error.

It also makes semantic sense, the order of testing is:

* check the status
* check the headers
* check the body

gabbi/driver.py

@@ -40,9 +40,9 @@ from gabbi import suite as gabbi_suite
 from gabbi import utils
 
 RESPONSE_HANDLERS = [
+    handlers.HeadersResponseHandler,
     handlers.StringResponseHandler,
     handlers.JSONResponseHandler,
-    handlers.HeadersResponseHandler,
 ]