Updating the escape regular expressions to fix Ruby 1.8 -KU mode bug.

In Ruby 1.8, when you are operating with unicode encoding, \w includes
UTF-8 characters such as ☃. Faraday therefore does not escape these
correctly. This patch takes from the regular expressions used by
@sferik in https://github.com/laserlemon/simple_oauth/blob/master/lib/simple_oauth/header.rb
when deciding what to should be escaped.