Allow application/x-www-form-url_encoded POST requests to use file objects as the request body

[catlee]

Description

Currently if you try to do something like

Faraday.post("https://httpbingo.org/post", StringIO.new("hello"), {'content-length': '5'})

you will get an ArgumentError raised from url_encoded.rb since it's trying to pass the StringIO object to Hash.

This PR skips over bodies that respond to the :read method in addition to bodies that respond to the :to_str method.

Todos

List any remaining work that needs to be done, i.e:

• [x] Tests
• [ ] Documentation