Adapter tests

Could use some feedback here.

There's a few things going on. I originally wanted to see what was causing [this annoying spec wart](https://github.com/seancribbs/webmachine-ruby/blob/99028bb3a761d6aa582bcc6df93c280fc8f1f788/spec/webmachine/adapters/reel_spec.rb#L27), realized that it was my fault, and set out to fix it.

The fault lies with [this spec](https://github.com/seancribbs/webmachine-ruby/blob/99028bb3a761d6aa582bcc6df93c280fc8f1f788/spec/webmachine/adapters/mongrel_spec.rb#L50). It's not a very helpful spec, but it did catch a [weird integration issue](https://github.com/seancribbs/webmachine-ruby/pull/38).

I decided to just make some better integration specs cover more of the adapter internals. This actually caught one issue with translating headers.

I had to change the production code in order to gracefully shut down the servers after the examples run. There might be a more clever way to do this, which wouldn't require adding an additional method to the mongrel/reel adapters' APIs.

Should I keep going with this?