Avoid CORS preflight requests on GET fetches.
Avoid CORS preflight requests on GET fetches.

This is a pretty easy change that should make things a bit snappier for
people further from the server, at least for data retrieval (removes 1
round trip of HTTP, so potentially more than 1 round trip of TCP).
Before we were always adding a few headers to every request, but these
turned GET requests into "non-simple" ones (pretty unnecessarily, the
extra headers weren't actually useful on them) that had to be
preflighted. This change just handles them more specially and only adds
an Accept header.

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