input: Write a function to create a simple chatbot that uses Dialogflow to answer questions about the current days of the week. The chatbot should be built using Flask and should handle both GET and POST requests at the root ('/') endpoint. If the session_id is not present, it should create one. The function should return the fulfillment text from Dialogflow.
  
  assert create_chatbot() == "Welcome to the chatbot! How can I assist you with the current days of the week?"
  `
output: 
  def create_chatbot():
      from flask import Flask, request, jsonify
      import uuid
      import dialogflow
      from google.cloud import dialogflow_v2 as dialogflow
  
      app = Flask(__name__)
  
      @app.route('/', methods=['GET', 'POST'])
      def chatbot():
          session_id = request.args.get('session_id') or str(uuid.uuid4())
          project_id = 'your-project-id'
          session_client = dialogflow.SessionsClient()
          session = session_client.session_path(project_id, session_id)
  
          if request.method == 'POST':
              query_input = dialogflow.types.QueryInput(text=dialogflow.types.TextInput(
                  text=request.json.get('text'),
                  language_code='en-US'
              ))
              response = session_client.detect_intent(
                  session=session,
                  query_input=query_input
              )
              return jsonify({'fulfillment_text': response.query_result.fulfillment_text})
          else:
              return "Welcome to the chatbot! How can I assist you with the current days of the week?"
  
      return app
  