Handle status_code 204 responses for non-DELETE methods (#45)

* Handle status_code 204 responses for non-DELETE methods

Stopping an agent is a PUT request that returns a 204.

* be defensive: return OK if a response content-type header is None

pybuildkite/client.py

@@ -72,7 +72,8 @@ def request(
         if with_pagination:
             response = self._get_paginated_response(response)
             return response
-        if method == "DELETE":
+        if (method == "DELETE" or response.status_code == 204 or
+                response.headers.get('content-type') is None):
             return response.ok
         if headers.get("Accept") is None or headers.get("Accept") == "application/json":
             return response.json()