Fix Sinatra logging.

Original commit[1] authored by @capitaomorte never seemed to make it into master.
This fixes a serious bug[2] that prevented any request logging with Sinatra
applications.

[1]
https://github.com/rtomayko/shotgun/commit/ce5d4994eb403c3a73146cffc31b40564fdf6291
[2] https://github.com/rtomayko/shotgun/issues/34