Proper fix for issue #4

It turns out that Net::HTTP::read_entity_body's documentation is not
100% accurate: When used with IO::Socket::SSL, it may return undef
and set $! to EAGAIN if "no data could be returned this time".
(This is pretty much what one would expect from a UNIX-like API, anyway.)
Merge pull request #12 from hillu/io-ssl-eagain-fix

Proper fix for issue #4