Use modern 'distinct' syntax in tests.

The old syntax is now an error: https://jira.mongodb.org/browse/SERVER-12642

Author: ajdavis

test/test_read_preferences.py

@@ -309,11 +309,11 @@ def test_command(self):
 
  # Distinct
  self._test_fn(True, lambda: self.c.pymongo_test.command(
- 'distinct', 'test', key={'a': 1}))
+ 'distinct', 'test', key='a'))
  self._test_fn(True, lambda: self.c.pymongo_test.command(
- 'distinct', 'test', key={'a': 1}, query={'a': 1}))
+ 'distinct', 'test', key='a', query={'a': 1}))
  self._test_fn(True, lambda: self.c.pymongo_test.command(SON([
- ('distinct', 'test'), ('key', {'a': 1}), ('query', {'a': 1})])))
+ ('distinct', 'test'), ('key', 'a'), ('query', {'a': 1})])))
 
  # Geo stuff. Make sure a 2d index is created and replicated
  self.c.pymongo_test.system.indexes.insert({