Support proxy credentials in http_proxy

Previously, the http_proxy support did not pass through credentials
from the environment variable if specified.  This limited the support
to proxies that did not require authentication.  This change adds
support for proxy URIs that contain credentials.

    cf set-env <app-name> http_proxy http://user:password@host:port

[#66924894][resolves #39]
Merge 66924894-proxy-credentials to master

[Completes #66924894]