Previously, the Actuator’s endpoints did not specify a produces
attribute on their request mappings. With Jackson’s XML binding on the
classpath, this would lead to requests made by a browser receiving
application/xml responses (due to the Accept header indicating that
application/xml is preferred). This was problematic as some of the
response payloads were not legal xml. Problems included XML tags
beginning with ‘\’ or containing ‘#’.

This commit updates the endpoints to specify that they produce
application/json. The environment and metrics endpoints have also been
updated so that always return a JSON object, even when they are
returning a single entry. This consistency avoids problems where
clients may not consider a single scalar value to be legal JSON.

Closes gh-2449