fix "already initialized constant" warnings

This pull request fixes "already initialized constant" warnings when using the google-api-client and faraday gems together. See: google/signet#6.

    [project_path]/.bundle/gems/faraday-0.8.8/lib/faraday/utils.rb:16: warning: already initialized constant KeyMap
    [project_path]/.bundle/gems/faraday-0.8.8/lib/faraday/utils.rb:173: warning: already initialized constant ESCAPE_RE
    [project_path]/.bundle/gems/faraday-0.8.8/lib/faraday/utils.rb:183: warning: already initialized constant DEFAULT_SEP

Tests are passing. 

Also - if there's any way to get these into the 0.6.x gem, everyone on my team would be very grateful :)