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

* v1.5:
  PHPLIB-485: Inherit collection's typeMap in distinct
  PHPLIB-481: Add functional test for pipeline-style bulk updates
  PHPLIB-481: Remove duplicated bulk write tests
  PHPLIB-481: Add the ability to specify a pipeline to an update within bulk write

PHPLIB-485: Inherit collection's typeMap in distinct

PHPLIB-481: Add the ability to specify a pipeline to an update within bulk write