fix(cache): cache key comparison ignores milliseconds

[hgrsd]

See #489. The comparison used for cache keys ignores milliseconds, leading to false positive cache hits.

In this PR I've changed the comparison strategy to compare the unix timestamps of the relevant dates instead. Let me know whether you think this is the right approach :)

Thanks for contributing to rrule!

To submit a pull request, please verify that you have done the following:

• Merged in or rebased on the latest master commit
• Linked to an existing bug or issue describing the bug or feature you're
  addressing
• Written one or more tests showing that your change works as advertised

[hgrsd]

Hello @jakubroztocil @lyschoening @davidgoli 👋 I'd be grateful for a review and your view on the proposed change :) Thanks in advance!

[hgrsd]

@jakubroztocil @lyschoening @davidgoli just checking in to see if you have had any time to take a look at this? Let me know!

[jkbrzt]

Thanks!

[hgrsd]

Thanks for merging! 😃 Do you have a sense of when this might be released? Anything I can do to help do that?