Retain default order in HttpMessageConverters

The original fix for gh-1293 (commit 05e6af23) caused test failures due
to the fact that Spring Boot's MappingJackson2HttpMessageConverter was
added before Spring's default StringHttpMessageConverter.

This commit changes the HttpMessageConverters logic so that additional
converts are added just before any default converter of the same type.
This allows additional converters to be added whilst still retaining
the sensible ordering of the default converters.

Fixes gh-1293
Polish