Check unknown error content type before read

This updates the error deserializer to check that the content type
of a response matches the protocol's content type before it tries
searching the payload for a shape id.

Author: JordonPhillips

packages/smithy-http/src/smithy_http/aio/protocols.py

@@ -183,7 +183,7 @@ async def _create_error(
             operation=operation, response=response
         )
 
-        if error_id is None:
+        if error_id is None and self._matches_content_type(response):
             if isinstance(response_body, bytearray):
                 response_body = bytes(response_body)
             deserializer = self.payload_codec.create_deserializer(source=response_body)
@@ -227,3 +227,8 @@ async def _create_error(
             is_throttling_error=is_throttle,
             is_retry_safe=is_throttle or None,
         )
+
+    def _matches_content_type(self, response: HTTPResponse) -> bool:
+        if "content-type" not in response.fields:
+            return False
+        return response.fields["content-type"].as_string() == self.content_type