Add RubyGems proxy support

[greeddj]

Hello

This PR adds support for using hub as a RubyGems source or mirror.

As source example:

source "http://localhost:6587/rubygems/rubygems"
gem "puma"

source "https://rubygems.org" do
  gem "rack"
end

As mirror example:

bundle config --local mirror.https://rubygems.org http://localhost:6587/rubygems/rubygems

Also in this PR fix UA header string

-- UserAgent
++ User-Agent