fix build_schema bug when model points to loaded ref

Author: p1-alexandrevaney

openapi_python_client/parser/responses.py

@@ -21,6 +21,7 @@ class Response:
 
 _SOURCE_BY_CONTENT_TYPE = {
     "application/json": "response.json()",
+    "application/problem+json": "response.json()",
     "application/vnd.api+json": "response.json()",
     "application/octet-stream": "response.content",
     "text/html": "response.text",