Use SSL with rubygems.org
Require date from stdlib in gemspec
Remove misplaced Client#stop call

The client is likely still receiving the response entity/body
when ::request returns. Calling #stop here will prematurely
close the StringIO representing the body, leaving no indication
of what happened.
Add SSL support to Client, sadly without tests
Avoid signature mismatch between callback and errback
Set fallback Host header if none is set
Fix rspec's warnings about double vs. stub