Merge lp://staging/~cjwatson/lazr.restful/double-closing-brace into lp://staging/lazr.restful

Fix double closing brace when encoding the result of a custom operation where the result has an adapter to ICollection.

Batch encoding is very weird; BatchingResourceMixin.batch intentionally omits the closing brace, and the caller is supposed to add it after possibly adding other parameters. This means that it's important for overridden implementations of the batch method in subclasses not to add the closing brace, as that violates the implied contract; only methods other than batch are allowed to add it. We were violating that rule in one code path, which resulted in a double closing brace.

This previously went unnoticed because of an overly-liberal doctest (which I had to fix anyway in order to cope with varying dict iteration order) and I think also because Launchpad tends to declare operations as having a return_type providing ICollectionField rather than directly returning something that can be adapted to ICollection.