Default to doing DNS resolution through a proxy server if present.

[micolous]

• Resolve an issue where proxy information by default was not resolving DNS through a proxy server (unlike the socks library), so users getting proxy information from an environment variable (HTTP_PROXY/HTTPS_PROXY) with no external DNS could not connect to hosts with DNS names. There isn't any way to set proxy_rdns from proxy_info_from_environment or proxy_info_from_url, so this gives a sensible default, making httplib2 work the same way as most web browsers.
• Resolve a potential issue where if the proxy server was a different address family to the destination host and there was functional DNS, may not work. For example, using a IPv4-only proxy server to connect to an IPv6-only host via an IPv4-only internal network. This code (and thus issue) only appears to exist in the Python 2 version of the code.
• Improved documentation of the ProxyInfo class.
• Because the default behaviour is now to resolve DNS names through a proxy server, this has the added benefit of preventing DNS leaking issues to an untrusted network when using a proxy over SSH or Tor.

[cliffano]

Verified that the proxy_rdns sensible default change works for solving dns resolution behind a proxy server and without a local nameserver.
@jcgregorio Any chance of releasing 0.10?

A bit of context: I'm using httplib2 via Ansible's uri module. The default proxy_rdns=None in httplib2 v0.9 results in sock.connect() to raise gaierror which in turn raises ServerNotFoundError, which Ansible uri module passes as a failure with "Unable to resolve host name given." error message.

Edit: typo and clarity.