Fixes applied to the request and targetting

This patch solves some  extracts the dictionaryValue function to a Helper (for possibility of being shared).
This patch adds nulllificaton of strong parameters in `PNAdRequest` and `PNAdRequestTargeting`
PNAdRequestParameters This patch adds some missing parameters (not related with the platform, but to be more API focused)
PNAdRequest: This patch adds the startRequest method to check if everything is ready for the request (since there is a method that allows the client to create the request without start, this method is needed)