Rename has_key-vs-lookup.rb in gemspec.

This file was renamed in 30e455d.
Merge pull request #220 from dprince/fix_gemspec_has_key

Rename has_key-vs-lookup.rb in gemspec.
fix warnings around class level variable access
include scheme in socket key

closes #223
make socket#connect a private method

see also #222
add Excon.stub_for

allows lookup of stub and/or nil
closes #209
Release 0.21.0
remove request/response info from HTTPStatusError message

This prevents Excon from inadvertently displaying sensitive information,
and producing large error messages when request[:body] is a String.

All HTTPStatusError exceptions provide access to the request and
response, and may be rescued and inspected for debugging purposes.

Closes #219