Fixes integration tests for RevisionMiddleware

RevisionMiddleware is tested by applying it as a decorator to various view
functions. Django's decorator_from_middleware() utility function does the trick
of converting a middleware class to a decorator. However, in projects that
include the middleware in the MIDDLEWARE_CLASSES setting, the wrapped view
function is processed twice by the middleware. When RevisionMiddleware
processes a function twice, an ImproperlyConfigured exception is raised.  Thus,
use of Django's definition of decorator_from_middleware() was preventing
reversion integration tests from passing in projects that include
RevisionMiddleware in MIDDLEWARE_CLASSES.

To avoid this problem, this commit redefines decorator_from_middleware() to
return a decorator that does not reapply the middleware if it is in
MIDDLEWARE_CLASSES.  @decorator_from_middleware(RevisionMiddleware) is then
used to wrap almost all RevisionMiddleware test views.
Merge pull request #285 from jmoldow/revision_middleware_decorator-fix

Fixes integration tests for RevisionMiddleware