Headers / encoding patch fix -- more significant changes required.

[Kevin Brown]

While attempting to resolve various open issues related to gadgets.io and
header passing / caching, it's become apparent that some rather significant
reworking of RemoteContentFetcher was in order. The attached patch attempts
to address these issues. I want to leave this up until Monday to give more
people a chance at feedback since this will cause a compatibility break on
the RemoteContentFetcher interface.