test gdbm each_key called without a block

* The aim of this test is to aid other implementations in being
  conformant with MRI.

* The existing test calls GDBM#each_key with a block,
* this commit tests that, when called without a block, the method
  returns an Enumerator, and does not assume there is a block,
* the 'gdbm' gem attempts to have identical behaviour to this
  implementation, but assumes there is a block.