flask wrap_wsgi should check propagate_exceptions instead of debug

[ThiefMaster]

        if wrap_wsgi is not None:
            self.wrap_wsgi = wrap_wsgi
        elif self.wrap_wsgi is None:
            # Fix https://github.com/getsentry/raven-python/issues/412
            # the gist is that we get errors twice in debug mode if we don't do this
            if app and app.debug:
                self.wrap_wsgi = False
            else:
                self.wrap_wsgi = True

I think this should actually check app.propagate_exceptions because this is what determines whether an exception should be re-raised (and thus propagate to WSGI middleware). This property defaults to app.testing or app.debug but if PROPAGATE_EXCEPTIONS is set explicitly, it always uses that value.