Previously, JacksonHttpMessageConvertersConfiguration would configure a
general-purpose MappingJackson2HttpMessageConverter only if there was
no existing MappingJackson2HttpMessageConverter in the application
context. This was problematic when a
TypeConstrainedMappingJackson2HttpMessageConverter bean was present.
Such a bean is only capable of performing conversion for a specific
type, and therefore is no substitute for a general purpose converter,
yet its presence was causing the auto-configuration of a general
purpose converters to be turned off. This would leave Spring MVC’s
default converter being used for application/json requests which would
not honour the user’s Jackson configuration.

This commit enhances @ConditionalOnMissingBean so that the annotation
can be used to specify one or more types that should be ignored when
searching for beans. This allows the
TypeConstrainedMappingJackson2HttpMessageConverter beans that are
published by Spring Data REST to be ignored such that the
general-purpose MappingJackson2HttpMessageConverter is still
auto-configured.

Fixes gh-2914