Improve test for per-request params encoder
Improve test for per-request params encoder

* No need for defining a classname in test mode
* Asserts that only `encode` method is invoked
* Asserts that the encoder actually receives correct params

AvaelKross/faraday,prcongithub/faraday,prcongithub/faraday,davoclavo/faraday,pvdb/faraday,coinbase/faraday,lostisland/faraday,stayhero/faraday,davoclavo/faraday,pvdb/faraday,stayhero/faraday,ngpestelos/faraday,coinbase/faraday,ngpestelos/faraday,AvaelKross/faraday,lostisland/faraday