accept float claims but round down to ignore them

[croensch]

This makes #488 work without having to set leeway.

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[bshaffer]

Wouldn't it be a better solution to just use microtime(true) instead of time() for the JWT validation?

Update: microtime requires that gettimeofday has an implementation. It seems like this is always the case, but it's possible this library is running on an obscure operating system without an implementation. Maybe we should wait to support microtime in a major version bump

[bshaffer]

This is a good solution for now, and we can look at using microtime in a future version