Add specs for more HTTP::Body classes

[janko]

This adds specs for Body::Fixed, Body::Chunked, and Body::Buffered, including the specs that assert memory allocations.

I also included two changes to the application code:

• Body::Buffered#each was removed, as from what I could see it's the same as the inherited Body::Readable#each
• Body::Buffered#empty? was updated to actually return true when #read would return nil (like the RDocs for Body::Readable#empty? say)

[coveralls]

Coverage increased (+0.5%) to 89.8% when pulling 547ea36 on add-more-body-specs into 9db36bc on master.

[coveralls]

Coverage increased (+0.5%) to 89.8% when pulling 547ea36 on add-more-body-specs into 9db36bc on master.

[ioquatix]

Thanks for your effort making more specs and fixing bugs.