input: Write a python function to create a user and return a JSON message using Flask.
  assert create_user({'name': 'John Doe', 'age': 30, 'address': '1234 Elm Street'}) == {'message': 'User created.'}
  assert create_user({'name': 'Jane Doe', 'age': 25, 'address': '5678 Oak Street'}) == {'message': 'User created.'}
  assert create_user({'name': 'Alice Smith', 'age': 40, 'address': '91011 Maple Street'}) == {'message': 'User created.'}
output: from flask import Flask, request, jsonify
  
  app = Flask(__name__)
  
  @app.route('/create_user', methods=['POST'])
  def create_user():
      user_data = request.json
      # Here you would typically save the user_data to a database
      return jsonify({'message': 'User created.'})
  
  if __name__ == '__main__':
      app.run(debug=True)