Merge branch '2.2.x' into 2.2.x-merge-up-into-2.3.x_iA5R4YuE

* 2.2.x:
  Fix wrong assertion (#2335)
  Fix handling of upserts during scheduling for deletion (#2334)
  Fix wrong handling for nullable fields in upsert and update (#2318)
  [2.2] Fix builds (#2319)
  Allow mixed value in $not operator (#2307)
  Fix errors with nullable typed associations (#2302)
  Fix using null values in partial filter expressions (#2300)
  Fix preparation of $elemMatch operators in queries (#2298)
  Fix query preparation when in elemMatch (#2299)
  Fix mapping of the nullable option for XML driver
  Fix documentation for uploadFromFile
  Correctly handle write concern specified in defaultCommitOptions (#2294)
  Fix invalid strict comparison when validating mappings
  Update storage-strategies.rst
  Update working-with-objects.rst
  Test serialization of lock/version fields
  Fix locking when ClassMetadata is unserialized