Merging headers is repeated in a few places.

Useful to have a common bit of code to handle that part.
Swapping in the newer header merging.

Using it for GET and POST.
Minor comments.
Renaming the gist module.

Renaming from a plural name to the singular, and updating the reference
to the module from the related unit tests.
Renaming the version.

Give them a VERSION to ask for instead.