DATAMONGO-1097 - Add support for $mul to Update.

We now support multiply on Update allowing to multiply the value of the given key by a multiplier.
DATAMONGO-1094 - Fixed ambiguous field mapping error message in BasicMongoPersistentEntity.

Original pull request: #245.
DATAMONGO-1096 - Use null-safe toString representation of query for debug logging.

We now use the null-safe serailizeToJsonSafely to avoid potential RuntimeExceptions during debug query printing in MongoTemplate.

Based on original PR: #247.

Original pull request: #251.
DATAMONGO-1101 - Add support for $bit to Update.

Prepare issue branch.
DATAMONGO-1101 - Add support for $bit to Update.

We now support bitwise and/or/xor operations for Update.