During testing, ensure we're not getting text/html unexpectedly.

src/allmydata/storage/http_client.py

@@ -443,11 +443,20 @@ class StorageClient(object):
             kwargs["data"] = dumps(message_to_serialize)
             headers.addRawHeader("Content-Type", CBOR_MIME_TYPE)
 
-        return await self._treq.request(
+        response = await self._treq.request(
             method, url, headers=headers, timeout=timeout, **kwargs
         )
 
-    async def decode_cbor(self, response, schema: Schema) -> object:
+        if self.TEST_MODE_REGISTER_HTTP_POOL is not None:
+            if response.code != 404:
+                # We're doing API queries, HTML is never correct except in 404, but
+                # it's the default for Twisted's web server so make sure nothing
+                # unexpected happened.
+                assert get_content_type(response.headers) != "text/html"
+
+        return response
+
+    async def decode_cbor(self, response: IResponse, schema: Schema) -> object:
         """Given HTTP response, return decoded CBOR body."""
         with start_action(action_type="allmydata:storage:http-client:decode-cbor"):
             if response.code > 199 and response.code < 300: