Set supports_pending_changesets for Plastic.

When I added the supports_pending_changesets flag, I neglected to add it for
Plastic. This broke things.

Fixes part of bug 3044.
Fix default value for HostingService._http_request headers parameter

The default value for the 'headers' parameter to the _http_request was None,
which meant that if something called it without passing in headers,
urllib2.Request()'s constructor would throw an exception. Other similar code
uses a default value of an empty dictionary, which is more appropriate.

Reviewed at https://reviews.reviewboard.org/r/4392/
Allow FileAttachment.file to store longer file paths.

FileField, by default, limits to 100 characters, which was barely enough
as it was. With the new change for prepending a hash, this was
suffocating.

We now set that max length to 512 characters. While that's far smaller
than the max path length of most file systems, it should be enough for
our use, and is far more than what users have had in the past.
Merge branch 'release-1.6.x' into release-1.7.x
Merge branch 'release-1.7.x'
Allow FileAttachment.file to store longer file paths.

FileField, by default, limits to 100 characters, which was barely enough
as it was. With the new change for prepending a hash, this was
suffocating.

We now set that max length to 512 characters. While that's far smaller
than the max path length of most file systems, it should be enough for
our use, and is far more than what users have had in the past.
Merge branch 'release-1.7.x'