Updated the behaviour of timeout and interDigitTimeout handling

[dmitrikozlov]

I aligned the behaviour with the VXML specification, which reflects industry practice.

The VXML 2.0 standard: https://www.w3.org/TR/voicexml20/
Appendix D — Timing Properties
D.1. DTMF Grammars
The "grammar" here is the rules defined by minDigits , maxDigits and numDigits.
The only difference is that there is no separate termtimeout parameter. The code uses interDigitTimeout as interdigittimeout and termtimeout.

The current implementation:

• keeps using timeout until minDigits reached
• can return an invalid input where the number of digits is less than minDigits