Zlib typo fix

This is a minor PR, but it cleans up a couple of stdlib failures. The outstanding failures are primarily due to lack of streaming deflate support.

Note the doc changes in #deflate between:

* http://ruby-doc.org/stdlib-1.9.3/libdoc/zlib/rdoc/Zlib/Deflate.html
* http://ruby-doc.org/stdlib-2.2.0.preview1/libdoc/zlib/rdoc/Zlib/Deflate.html

Notably, the default flush mode is now Z_FINISH, and if a block is passed to deflate, it's Z_NO_FLUSH, and each deflated chunk is yielded to the block.