Compression: Properly close gzip/deflate.

The previous patch did not work properly work, if the response was
written using several writes.

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.

Conflicts:
	compress.go
Format and fix duplicate method definition
Merge pull request #668 from revel/klauspost/close-gzip/deflate

Compression: Properly close gzip/deflate