R1:5215eb306791

Fix an issue where unexpected debugging output would run afoul of automatic compression

Summary:
If you put "echo" or "print" statements into the code at random places (as I frequently do during development), they would emit before we enabled compression.

This would confuse the compression mechanism and browser. I tried using `headers_sent()` to selectively disable compression but that didn't appear to fix this interaction (I think emitting this text does not cause headers to send, but does let contet…