Validate verb+endpoint combinations before the HTTP call is fired

[igorsantos07]

Not all endpoints have all operations available. This was solved for the Webhooks endpoint as it needs some custom methods as well, but it would defeat the purpose of the magic calls happening in the library for other endpoints.

Instead, on debug mode (i.e. Config::debugLevel(DEBUG_BASIC)) we should run conservative validation rules to warn the developer the call they're trying to run is not available on the API. There are already specifications for that (on BaseEndpoint::$specs) but that's not yet used.

That's more friendly than a random 404 error with no specification on what went wrong from ProsperWorks.