Spec tests from mongodb/specifications@60adafcffa07174ac54c6e8658bd37a3faf350c3

Server versions < 3.4 do not support write concerns for drop commands. Rather than pass the context's write options directly to drop(), filter them through the Collection class so it can decide to inherit them based on the server's wire version.

Likewise for executing find() on the output collection.

* v1.5:
  PHPLIB-494: Fix validation of options with default values
  PHPLIB-494: Test against null for options with default values
  Simplify generator setup in tests
  PHPLIB-501: Use IteratorIterator to prevent memory leak with generators