Fix forwarding of headers starting with http_ in the proxy

[ejennings-mdsol]

Having talked with @johngluckmdsol offline about #11, this PR is inspired by his fix and from his discussion with @cabbott. It seems like when the proxy was originally created in https://github.com/mdsol/mauth-client/pull/45 that the intent was to forward headers that begin with HTTP_ but to remove the leading HTTP_ (but to not forward HTTP_HOST). This PR restores that behavior that was changed in https://github.com/mdsol/mauth-client/pull/106.

@jfeltesse-mdsol

[johngluckmdsol]

This spec is a great start but it's not a substitute for an integration test that would have actually caught the original problem. If we add request headers to mauth and the proxy mangles them again, this won't catch that. Since we don't modify mauth that often, it's fine, so LGTM

[jfeltesse-mdsol]

Ahhhh it all makes sense now.

When @johngluckmdsol first brought it up I was confused by the lack of any match or =~ call that would be needed for $' to do anything useful.
As you pointed out, this is the offending refactoring that took place many moons ago:

- request_env.each do |k,v|
-  if k =~ /\AHTTP_/ && !%w(HTTP_HOST).include?(k)
+ request_env.each do |k, v|
+  if k.start_with?('HTTP_') && !%w(HTTP_HOST).include?(k)
    name = $'

I guess this serves as another lesson that ruby global variables are evil.

[jfeltesse-mdsol]

merged and tagged but I don't have access to rubygems.org to publish a new release...