title: Allow CORS to flask endpoint
body: <p>I have developed a flask API and am attempting to send a POST request to it from a react app. However I keep receiving this error message: </p>
  
  <pre><code>Access to fetch at 'http://localhost:5000/api/project/' from origin 'http://localhost:3000' has been
  blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
  If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource 
  with CORS disabled.
  </code></pre>
  
  <p>Here is my fetch request in react:</p>
  
  <pre><code>fetch('http://localhost:5000/api/project/', {
        method: 'POST',
        mode: 'cors',
        body: JSON.stringify(project),
        credentials: 'include'
      }).then(function(response) {
        return response.json();
      }).then(function(data) {
        console.log(data)
      });
  </code></pre>
  
  <p>Here is my function for the endpoint:</p>
  
  <pre><code>    @cross_origin( supports_credentials = True )
      def get( self ):
          # Make sure user is logged in
          project_owner = session.get( 'username' )
  
          if project_owner is None:
              return response.cannot_process( ERROR_MSG.LOGIN_ERROR_MSG )
  
          # Establish db connection
          database_conn = db_mod.get_db()
          if database_conn is None :
              logging.error( ERROR_MSG.DB_ERROR_MSG )
              return response.service_unavailable( ERROR_MSG.DB_ERROR_MSG )
  
          # Try to get 'user' collection     
          user_collection = database_conn.get_collection( 'users' )
          if user_collection is None:
              logging.error( ERROR_MSG.COLLECTION_NOT_FOUND('users') )
              response.cannot_process( ERROR_MSG.COLLECTION_NOT_FOUND('users') )
  
          # Try to get 'projects' collection
          project_collection = database_conn.get_collection( 'projects' )
          if project_collection is None:
              logging.error( ERROR_MSG.COLLECTION_NOT_FOUND('projects') )
              response.cannot_process( ERROR_MSG.COLLECTION_NOT_FOUND('projects') )
  
          # Get owner document
          user_document = database_conn.get_document_by_key( user_collection, project_owner )
          documents = []
          # First find out all the owner projects
          owner_edge = database_conn.get_collection( 'project_ownership' )
          if owner_edge is not None:
              # Now get the owner project list
              owner_project_list = database_conn.get_edges_out( owner_edge, user_document )
              for owner_project in owner_project_list:
                  owner_documents = database_conn.get_documents_by_template( project_collection, {"_id": owner_project.get("_to")} )
                  if owner_documents is not None: documents.extend( owner_documents )
  
          # Next find out all the collaborator projects of this user
          collaboration_edge = database_conn.get_collection( 'project_collaboration' )
          if collaboration_edge is not None:
              # Now get the collaborator project list
              col_project_list = database_conn.get_edges_out( collaboration_edge, user_document )
              for col_project in col_project_list:
                  col_documents = database_conn.get_documents_by_template( project_collection, {"_id": col_project.get("_to")} )
                  if col_documents is not None: documents.extend(col_documents)
  
          project_data = []
          for document in documents:
              # Classification level is stored as a number, convert back to a string
              document['classification'] = fromClassificationLevel( document.get('classification') )
              project_data.append( marshal(document, project_data_model, skip_none = True ) )
  
          return response.ok_json( jsonify( {'result': project_data} ) )
  </code></pre>
  
  <p>I am unsure of how to set the access-control-allow-origin header from my API endpoint? I am currently using flask_cors module's @cross_origin() decorator for my endpoint. This works for my GET requests, but I am receiving this error for my POST request. Any idea how to solve this? 
  Thanks</p>