fs: fix .write() not coercing non-string values

[Fishrock123]

Should fix #1098

relevant docs: https://github.com/iojs/io.js/blob/v1.x/doc/api/fs.markdown#fswritefd-data-position-encoding-callback

R=anyone

[Fishrock123]

test is just adapted from test-fs-write.js

[brendanashworth]

LGTM. Thanks @Fishrock123, merged in cf565b5.

[rvagg]

@brendanashworth since @Fishrock123 is a collaborator it's probably best to let him merge it. I personally would have preferred a bit more time to have some more eyes on this before merging as it's not as trivial as a doc fix.

[brendanashworth]

@rvagg sorry, you're right, I probably should've let it sizzle - then I buried it with another commit. Won't happen again :)

[Fishrock123]

The behavior appears to have been incorrect from the start: 7ca77ea#diff-9a205ef7ee967ee32efee02e58b3482dR484

No tests were added when the API was added, and thus it was not caught.