Add specs for Request#has_body?.
Add LazyRequestBody#empty? method.

This unbreaks Request#has_body? if the body is a LazyRequestBody.
Revert "Add another failing test case for Rack and content-type."

Since the rack adapter uses Rack::Response now, we can revert this.
@pawelpacana confirmed in issue #98 that this is safe.

Unbreaks tests with rack >= 1.5.

This reverts commit f33677849d0c1cad8362a625d17447b87fe56289.
Merge pull request #98 from seancribbs/has_body_fix

Fix Request#has_body? with a LazyRequestBody