TestGCSIO.test_last_updated (gcsio_test.py) fails when the current timezone offset and the timezone offset on 2nd of Jan 1970 differ

The test TestGCSIO.test_last_updated uses timestamp 123456.78 as the last updated timestamp.  This timestamp is converted into a naive datetime in the Fakefile class get_metadata method (gcsio_test.py line 72) Then in the GcsIO class last_updated method (gcsio.py ) is converted back to timestamp. But the conversion is incorrect when the the timezone offset is different in 1970 and now. In my case now Singapore is GMT****8 and it was only GMT****7:30 in 1970. So the test fails. 

Imported from Jira [BEAM-6947](https://issues.apache.org/jira/browse/BEAM-6947). Original Jira may contain additional context.
Reported by: csabakassai.