fix proxy var issues

- if the env var is already set, leave it alone
- look for '://' to see if we were given a scheme or not
  (https_proxy may be passed an 'http://' url)
add spec tests for http_proxy fixes
add notes on bugfixes to CHANGELOG
Merge pull request #1832 from opscode/lcg/proxy-var-fixes

Lcg/proxy var fixes