Fix method declaration to be ruby-1.8-compatible

[anujdas]

Everything else in this gem seems to work fine in ruby 1.8.7 save for this one declaration, since the 1.8 parser can't deal with non-optional arguments after optional ones. I don't think that 1.8-compatibility should be a goal necessarily, but in this case it's pretty easy to add and would help me out ;)

[waterlink]

Hi @anujdas. This repo has moved to https://github.com/waterlink/rack-reverse-proxy

I will merge your branch manually, because I have recently added 1.8.7 to travis.yml and got the same error :)

[waterlink]

Done. Now you can use github: "waterlink/rack-reverse-proxy" and, when new version will be released - from rubygems.