Previously, HypermediaAutoConfiguration would trigger the creation of
an ObjectMapper bean named _halObjectMapper. This bean did not have the
spring.jackson.* configuration applied to it, however its presence
would revent JacksonAutoConfiguration from creating its
ObjectMapper. This left the user with an ObjectMapper that did not
honour the spring.jackson.* configuration.

This commit updates HypermediaAutoConfiguration to use the
Jackson2ObjectMapperBuilder that may have been created by
JacksonAutoConfiguration. If the builder exists it is used to configure
the _halObjectMapper bean.

Fixes gh-1949