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`.