Previously, TestRestTemplate applied the root URI to URIs by
converting them to a String and then passing the String to the
RestTemplate delegate. Being a String, meant that the URI passed
through RestTemplate's standard URI template expansion processing
using the configured UriTemplateHandler. While this caused the root
URI to be applied, it also had the unwanted side-effect of
encoding the URI for a second time.

This commit updates TestRestTemplate so that, when configured with a
RootUriTemplateHandler, it applies the root URI directly and then
passes a modified URI to the RestTemplate delegate. Being a URI means
that no template expansion is performed and the possible double
encoding is avoided.

Closes gh-8163