* add initializer on Faraday::Adapter that accept and store a block with custom configuration
use the `@ config_block ` block on `net_http` and `net_http_persistent` adapters
added tests
* fixes warning in tests
patron was supporting custom block already! Small refactoring to conform to other adapters
* added implementation for httpclient
excon and EM adapters now all support `connection_options` in the initializer
* additional test for httpclient adapter