Add specs for streaming in inflate processing

In ruby 2.x zlib library has support for streaming in inflate
processing. I added specs for new API of #inflate and #deflate methods.
Each of these methods may take a block which can process one chunk of the
stream.

Commit from ruby:
https://github.com/ruby/ruby/commit/3cf7d1b57e3622430065f6a6ce8cbd5548d3d894
Merge pull request #2 from LTe/tests-streaming

Add specs for streaming in inflate processing