(#5838) Added support for HEAD requests to the indirector.

Added the ability for the indirector to handle REST HEAD requests.
These are done using a new indirector method, head(), which should
return true if find() would return a result and false if find() would
return nil.

Access control for the head method is the union of that for the find
and save methods. That is, if either find or save is allowed, then
head is allowed. This is necessary so that users will not have to
change their authconfig to take advantage of the new feature.

Paired-with: Jesse Wolfe <jesse@puppetlabs.com>