Fix regressions in FormReader / FileBufferingReadStream.

[Tratcher]

#605 @davidfowl @muratg @benaadams @BrennanConroy

The recent perf changes caused two regressions in the FileBufferingReadStream used by form reader. The one reported in #605 is that most of the data buffered in memory gets discarded when transitioning to the file buffer. This happened because Write was accidentally given the wrong length when I cleaned up some of @benaadams's code.

The other regression was that the constructor only rented buffers if the in-memory threshold was less than 1mb, but Read always assumed it was using rented buffers. This would have caused ArgumentNullExceptions trying to use an uninitialized buffer. The fix is to rent a buffer when needed and do a manual CopyTo loop.

Compare with bd60507#diff-6bee66e850efacd8fe96daae334fa1c5R192

[rynowak]

It breaks my brain a little bit to look at this code and see 'buffer' used for both Stream and arrays

[BrennanConroy]

Looks reasonable