(PUP-3352) Normalize HTTP Accept-Encoding values

Previously, puppet and the module tool uses different Accept-Encoding
values. The puppet version gave equal weight to gzip and deflate,
whereas the module tool preferred gzip. The module tool's version
appears to have been copy and pasted from ruby's Net::HTTP code.

This commit changes puppet to give higher precedence to gzip (to match
the others), and refactors the value of the header into a constant that
both puppet and the module tool use.