text: Title: 400 error in Swagger when using POST/PUT through reqparse
  Body: Hey all,
  
   While testing out PUT/POST requests using reqparser through Swagger UI (using _**Try it Out!**_), my application will throw a 400 error with the following message:
  
  `{
    "message": "The browser (or proxy) sent a request that this server could not understand."
  }`
  
  The same call will result in a success when submitted through Postman however. There is no stacktrace for the error. Also note that this issue only arises through passing the reqparse through @api.expect()
  
  I can successfully pass a model through without any error calling the api on swagger. However, I need the option to pass things like choices etc for the user.
  
  I'm using Flask-restplus v 0.10.0 and Python v 3.6. My SQL is handled through pyodbc 4.0.23. 
  
  Here is the code I use for setting up the reqparser:
  
  ```
  parser = reqparse.RequestParser()
  
  parser.add_argument("alternateNameId", type=int, required=False)
  parser.add_argument("alternateName", type=str, required=True)
  parser.add_argument("isColloquial", type=bool, required=True, default='False')
  parser.add_argument("isSearchTerm", type=bool, required=True)
  ```
  
  and then it's called through the @api.expect decorator as follows:
  
  ```@api.route('/<int:diseaseId>/AlternateName', methods=['PUT'])
  class AlternateName(Resource):
      @api.doc(model=altNameModel, id='put_alternatename', responses={201: 'Success', 400: 'Validation Error'})
      @api.expect(parser)
      @auth.requires_auth
  
  
      def put(self, diseaseId):
  ```
  And here are screenshots of the swagger UI:
  ![2018-10-29_9-24-14](https://user-images.githubusercontent.com/35080007/47652648-dd3c7080-db5c-11e8-875a-546286a49f51.png)
  
  ![2018-10-29_9-23-41](https://user-images.githubusercontent.com/35080007/47652581-aa927800-db5c-11e8-98ba-952888466cc4.png)
  
  
  I have seen similar issues logged but nothing quite addressing the fact that the operation only fails through the swagger UI and a GET request operates as normal.
  
  Has anyone seen this behavior before or understand how to mitigate it? My users would be using swagger as their main UI to access the endpoint.