For mysterious reasons, the existing string was just wrong: it was missing a colon.
There is no apparent reason for this discrepancy.

This should be safe to fix because existing uses would not be RFC822-compliant;
people cannot be depending on it to generate correct mail headers.

Fixes #3444.

R=golang-dev, dsymonds, iant, rsc
CC=golang-dev
https://golang.org/cl/5969072