Port to ruby 2.0

ruby-bugzilla doesn't work ruby 2.0 or later. Because it hit two issues.

1. since 2.0, respond_to?(name) no longer return true if "name" point to protected method.
    (i.e. incompatible change)
    see https://bugs.ruby-lang.org/issues/7743
2. XMLRPC no longer works if BZ server support http compression. (it's unfixed bug)
    see https://bugs.ruby-lang.org/issues/8182

This pull request fixes both.