Millisecond truncation is incorrect

[Cynthia]

Hello, I was testing my TOML parser/encoder against this test suite, and was surprised to find one of the datetime tests for the encoder was failing. After looking at it a bit, and at the repo's README (bold added by me):

Millisecond precision (4 digits) is required for datetimes and times

This seems seems incorrect. Millisecond precision is 3 extra digits (since 1000ms = 1s), which means this fail report seems invalid to me and is asking for sub-millisecond precision:

FAIL valid/datetime/milliseconds
     Values for key 'utc1' differ:
       Expected:     1987-07-05 17:45:56.1234 +0000 UTC (time.Time)
       Your encoder: 1987-07-05 17:45:56.123 +0000 UTC (time.Time)

[Martin Tournoij]

Thanks; I fixed it.