Use "name = value" consistently for headers

There has been some inconsistency in the format used for headers, with some
using "name: value" and others using "name = value". As these are only
approximations, either is fine, but internal consistency is desirable.

With the colon-prefixed names now extant, ":name = value" is subjectively
clearer than ":name: value"; for this reason, I have normalised headers to
"name = value" rather than "name: value".