Fix faked-out account GET for JSON and XML.

If account autocreation is on and the proxy receives a GET request for
a nonexistent account, it'll fake up a response that makes it look as
if the account exists, but without reifying that account into sqlite
DB files.

That faked-out response was just fine as long as you wanted a
text/plain response, but it didn't handle the case of format=json or
format=xml; in those cases, the response would still be
text/plain. This can break clients, and certainly causes crashes in
swift3. Now, those responses match as closely as possible.

The code for generating an account-listing response has been pulled
into (the new) swift.account.utils module, and both the fake response
and the real response use it, thus ensuring that they can't
accidentally diverge. There's also a new probe test for that
non-divergence.

Also, cleaned up a redundant matching of the Accept header in the code
for generating the account listing.

Note that some of the added tests here pass with or without this code
change; they were added because the code I was changing (parts of the
real account GET) wasn't covered by tests.

Bug 1183169

Change-Id: I2a3b8e5d9053e4d0280a320f31efa7c90c94bb06
Merge "Fix faked-out account GET for JSON and XML."