Incorrect return type for RRule.after

[jkepps]

RRule.after docs say that it returns Date or null, but the function definition specifies only Date. The function will return null if there are no dates in the recurrence after the given parameter, so the function does indeed return Date or null. The function definition should be updated to specify the correct return type.

The repro steps are fairly straightforward. Instantiate a new rrule object with a recurrence in the past. Invoke the after method with a date that's in the future. The function will return null.

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• Code sample reproducing the issue. Be sure to include all input values you
  are using such as the exact RRule string and dates.
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• Actual output
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• Your operating system
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  of the machine showing the bug)

[ghost]

I believe this should have been closed after #561