argument validation in request_spec

Provide addition specs for testing [validate method](https://github.com/rusikf/httparty/blob/master/lib/httparty/request.rb#L330).
Usually, ```HTTParty::RedirectionTooDeep``` exception appears when there are a lot of redirections( [and here is a test in cucumber](https://github.com/rusikf/httparty/blob/master/features/supports_redirection.feature#L18)), but additional testing it internally on Request, which tests [this line](https://github.com/rusikf/httparty/blob/master/lib/httparty/request.rb#L331), is nice , i think.