chore: Fix invalid unit test (#348)

With the latest release, pyRFC3339 allows a more flexible format. Prior
versions required an explicit T separator before the time section even
though the spec allows for a space.

While we still support the older version of pyRFC3339, we only test
against the latest version, and can safely update this invalid test.

Both of these are considered valid formats:

    1970-01-01T00:00:02.500Z
    1970-01-01 00:00:02.500Z

Author: keelerm84

ldclient/testing/impl/test_operators.py

@@ -57,7 +57,7 @@
         ["before", "1970-01-01T00:00:00.500Z", 1000, True],
         ["before", True, 1000, False],  # wrong type
         ["after", "1970-01-01T00:00:02.500Z", 1000, True],
-        ["after", "1970-01-01 00:00:02.500Z", 1000, False],  # malformed timestamp
+        ["after", "1970-01-01 00:00:02.500Z", 1000, True],
         ["after", "1970-01-01T00:00:02+01:00", None, False],
         ["after", None, "1970-01-01T00:00:02+01:00", False],
         ["before", "1970-01-01T00:00:02+01:00", 1000, True],