Sorter#_merge() not closing streams correctly.

[tjoneslo]

In the method Sorter#_merge(), the list of data readers is never closed. Which means the finally block where the input files are deleted, the files won't be deleted until the GC finally notices the open files are no longer used, then closes them and deletes the files.

I'm using the merge-sort as part of a much large, long running process which means the files may be left open, and on disk, for a significant period of time.

[cowtowncoder]

Regrettable oversight. Thank you for reporting this, will fix for next release.

[cowtowncoder]

Ok: although underlying readers should close on end-of-input (at least std DataReader implementations do), I added appropriate close logic for merged streams as well.
If you can build locally from master branch, test this out, that would be great: I hope to release 0.8.1 soon, assuming this fix works.