Previously, TestRestTemplate would only apply the UriTemplateHandler
to Strings and not to URIs. When using the auto-configured
TestRestTemplate, this prevented relative URIs from being
made absolute using LocalHostUriTemplateHandler.

The commit updates TestRestTemplate to turn URIs into Strings before
passing them to the delegate RestTemplate. Turning them into Strings
ensures that the delegate calls the UriTemplateHandler.

Closes gh-7891