ValueStringBuilder buffer clearing

[idg10]

Dispose should clear the buffer if it still owns the buffer by then.

ReturnRentedBuffer should provide a flag specifying whether the buffer should be cleared.

[mwadams]

Observations:

1. All code paths that call Dispose() directly have taken copies of the underlying buffer. Therefore the original before can always be cleared in Dispose().
2. To avoid breaking changes we should add an additional API for ReturnRentedBuffer() that takes the 'clear' flag, and reimplement the original method in terms of the new one.

[idg10]

All code paths that call Dispose() directly have taken copies of the underlying buffer. Therefore the original before can always be cleared in Dispose().

...or have taken ownership of it, so _arrayToReturnToPool will already be null.