Connection should use Faraday adapter specified in options to Restforce client

Currently, connection always uses Faraday.default_adapter even though I've supplied other adapter as an option to Restforce client: ```Restforce.new(:adapter => :typhoeus)```. It looks like a bug, since ```config.rb``` has such option defined with a default value equaling to ```Faraday.default_adapter```.

I've updated the ```connection.rb``` to take adapter from ```options``` and included a test.