Evict GrailsParameterMap objects before building flow execution url

The webRequest params used to build the flow execution url can contain nested GrailsParameterMap objects if the "params" attribute (of a redirect call in a controller, for instance) contains structured keys, e.g., a redirect to the "customerFlow",

  redirect(action:customer, params: ['customer.id': 123])

In this case, when the params attribute is parsed a synthetic GrailsParameterMap object will be created and added to the controller's params map with the key/value, customer: [id:123]. These synthetic GrailsParameterMap parameters should not be included in the resulting flow execution URL as they can interfere with the original parameters, 'customer.id' in this case, when the flow execution URL is parsed by the flow execution engine.