Fix #472 Use native http/https.Agent

The 'keep-alive-agent' module is not functional on Node v0.11.6 and
above due to http agent class refactoring.  While this breakage is
arguably a bug in Node, there's also no need for a keep alive agent in
Node v0.11 and above, since http keepalive is supported natively now.

Sniff for the Node version, and choose the appropriate agent and configs
accordingly.
test: update client malformed test for node v0.11

The 'spaces in path' error is now caught early in the client,
since it's never valid HTTP.  Update the test to handle this.
Merge pull request #475 from isaacs/gh-472

Fix #472 Use native http/https.Agent