[PATCH] reduce debug level on bodypipe re-write change

This critical-level debugs line has been pointed out by a few people.
It shows up whenever there is an existing BodyPipe on URL-rewriting (3xx
redirects avoid it).

As far a I can see there is no reason for having it as critical. Unless
anyone objects or can sow a reason I'll commit this in a few days.

I'm open to a better message if a reason for keeping it critical is known.

Amos

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