Have `rate_limit` return maximum values of `rate.Inf` (`math.MaxFloat64`) instead of `"inf"`

[chrisberkhout]

This will stop any problems caused by clients not processing "inf" values. An example of the currently required processing is in qualys_vmdr.

From a discussion of this issue in a PR:

Condition the return values from the rate limit extensions so that if either of these end up being infinite, it gets replaced with rate.Inf.

I think this would be backwards compatible; the relevant text in the documentation is

The map returned by the policy functions should have "rate" and "next" fields with type rate.Limit or string with the value "inf", a "burst" field with type int and a "reset" field with type time.Time in the UTC location. The semantics of "rate" and "burst" are described in the documentation for the golang.org/x/time/rate package.

This would remain true, but the second option would never happen.