Inconsistent behaviour for 'path' in URI constructor

[Jonathan Lange]

If you construct a URI like this:
   uri = URI(scheme='http', host='example.com', path='foo/bar')

You will end up with a URI like this:
   print str(uri)
   http://example.comfoo/bar

I'd expect a URI like:
   http://example.com/foo/bar

Or at least an error, asserting that the path should begin with a leading '/'. This is what other parts of lazr.uri do with paths without leading slashes. I'd much prefer implicitly adding one, but at least lazr.uri should be consistent, whatever it does.