Fix https://github.com/couchbase/couchbase-lite-java-core/issues/279

https://github.com/couchbase/couchbase-lite-java-core/issues/279 is proximally caused by a failure to close the content stream in attachmentAsserts. So this fixes that. But of course https://github.com/couchbase/couchbase-lite-java-core/issues/210 is the real issue!! :)

I also threw in a fix for the same issue in failingTestPullerGzipped just in case at some point it gets re-activated again.