Bugzilla: use 'header set' vs. 'header add'

set:

"The response header is set, replacing any previous header with this name."

add:

"The response header is added to the existing set of headers,
even if this header already exists.
This can result in two (or more) headers having the same name.
This can lead to unforeseen consequences, and in general set, append
or merge should be used instead."

from: http://httpd.apache.org/docs/2.2/mod/mod_headers.html

Change-Id: I3b28b72518c80d201126d6e7758c68133d95d6e0
Merge "Bugzilla: use 'header set' vs. 'header add'" into production