force the configured middlewares to be a Set

You shouldn't add the same exact middleware args multiple times and
this ensures that.
set the logging middleware like any other middleware

This pushes the logging middleware args onto the server config
middlewares set.  This means it is added just like any other user
config'd middleware is added.

This has the added benefit of testing that the sinatra app applies
the server config's middlewares.
Merge pull request #61 from redding/kr-middlewares

Middleware handling updates