Compatibility with Faraday 0.9

Faraday 0.9 (currently in release candidate) has updated its handling of options, and will throw an error if an option is passed in that it doesn't understand.

Koala currently passes its internal http_options hash (merged with any user options) directly to Faraday, including the use_ssl option which is not used by Faraday, resulting in this error:

    NoMethodError: undefined method 'use_ssl=' for #<Faraday::ConnectionOptions:0x00000006758f18>

I've added a filter method to select only the options that can be provided to a Faraday connection and pass those along to the Faraday initializer. Hope this is suitable.