storage.Key.get_contents_to_filename: sync mtime with 'updated' time from metadata

[tseaver]

Split from #148.

See the v1 API resource representation for Object, which defines the updated field as an RFC 3339 timestamp string (YYYY-MM-DDThh:mm:ssZ or YYYY-MM-DDThh:mm:ss+hh:mm).

[dhermes]

@tseaver and @grapefruit623 Please re-open if this was not fully fixed by #468.