Fix for sporadic truncation of output files due to incomplete I/O

[pjeby]

Hi. I'm using htmltidy as part of a regression test, to compare output of a system with its previous output. On numerous occasions, one file out of hundreds processed will be truncated somewhere in the middle. After extensive analysis and experimentation, I determined that the problem seems to be due to Node sometimes firing the subprocess exit event shortly before the last data event from the subprocess's stdout. (This is on Windows 7, by the way.)

This one-line change fixes the problem for tidy() calls, by delaying the final result callback until Node has finished processing all pending I/O, thus ensuring that the remaining data event has a chance to fire before the final callback is invoked.

Without the patch, an average of about 1 (randomly selected) file per two thousand processed is truncated on my computer. But with the patch, none are ever truncated as far as I can tell.

(The build is a complex one, with lots of other streaming events going on left and right, so I imagine the event ordering is a little less deterministic than in simpler processes using htmltidy, or that are not invoking it hundreds of times in rapid succession.)