close io object when fixture writing has completed

[tagCincy]

So typically, when passing a block to File.open, the IO object with automatically close when the block terminates. However, we have notice when attempting to write fixtures for largish database dumps that the IO stream is closing before the last few strings are written.

I believe the cause of this is here:

https://github.com/mbleigh/seed-fu/blob/master/lib/seed-fu/writer.rb#L96-L98

Basically, the ensure is killing the the stream before the last chunks have been written to the file by niling it.

I am not sure this solution is the best, but it solved our issue. I might take a stab at refactoring this code later.