Fixes a crash in CBLMultipartDocumentReader.m caused by the stream outliving the CBLMultipartDocumentReader that is its delegate. Solved by addiing a strong reference to the reader in its completion handler and breaking the retain loop in the end.
Fixes a crash in CBLMultipartDocumentReader.m caused by the stream outliving the CBLMultipartDocumentReader that is its delegate. Solved by addiing a strong reference to the reader in its completion handler and breaking the retain loop in the end.

mz2/couchbase-lite-ios,mz2/couchbase-lite-ios,mz2/couchbase-lite-ios