Restore Rack::Server#middleware backward compatibility

Fixes regression in #706. It intends to retain backward compatibility
with calls to #middleware by aliasing the method to the new
default_middleware_for_environment, but since it doesn't call #middleware
internally, subclasses that overrode it no longer work.

Restored compatibility by calling #middleware internally and delegating
its implementation to the new default_middleware_for_environment methods.
Merge pull request #729 from jeremy/fixes/default-middleware-override-regression

Restore Rack::Server#middleware backward compatibility