Compression: Properly close gzip/deflate.

The previous patch did not work properly work, if the response was
written using several writes. The writer would close after the first write had completed.

I couldn't find any way to have the close happen within the filter itself, so the revel server itself had to be modified slightly.

This calls close on the writer after it has been applied. Still a bit
fragile, but should work unless another filter overrides the
responsewriter after compression.