Following the changes made to combines the /links and /hal endpoints
into a single /actuator endpoint, a web browser accessing /actuator
would receive a 406 response if HAL browser was not on the classpath.
This commit updates the /actuator main entry point so that it will
serve JSON to a web browser when HAL browser is not on the classpath.
The actuator's embedded documentation has also been updated to reflect
the recent changes.
Closes gh-3696
dad0574fd5