Decode compressed JSON MIME bodies in multipart server responses

Support Gzip Content-Encoding of a document body encoded in a MIME
part (as when downloading a doc with attachments.)
For couchbase/sync_gateway#238
Fixed an assertion-failure when a _bulk_get download can’t parse a JSON doc

Fixes #232
Fixes #271
Fixed another assertion-failure when a _bulk_get download can’t parse a JSON doc

Found a piece that was missing from the prior fix 24f6381. The same assertion failure was occurring,
under slightly different circumstances.
Fixes #288 (speculatively). Also see #271, #232.
Finally fixed the multipart JSON-decoding bugs

Fixes #288
Fixes #289

Conflicts:
	Source/CBLMultipartDocumentReader.m
commented out unit test because compiler was complaining about it