Audit Log API Returns Non-Standard Timestamp

[lindluni]

The GitHub Audit Log API return a Unix timestamp in milliseconds, which the current timestamp unmarshalJSON implementation in go-github does not take into account. An implementation needs to be added to account for the audit log having a millisecond-based timestamp.

[gmlewis]

Thank you, @lindluni ! This could be handled probably in several different ways... I might simply modify github/timestamp.go to check if the parsed timestamp is "sane", and if not, see if parsing it as if it were milliseconds since epoch returns a "sane" timestamp... but I'm open to other ideas.

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