Improved support for HttpMessageConverters

[dschulten]

Currently we respond with Content-Type: application/json for HAL. That does not seem quite right, but I am not sure how to fix that.

Normally one would have a HttpMessageConverter for a mime type. But since we tweak Jackson to produce HAL over MappingJacksonXXXHttpMessageConverter, things are more difficult. I am also not sure if that is a good approach in the long run.

It might be more appropriate to have a HalMappingJackson2HttpMessageConverter and a HalMappingJacksonHttpMessageConverter which could be configured like any other message converter if you want HAL support. That MessageConverter should do the Jackson tweaking internally.
If you agreee, I could look into that.

[gregturn]

Superseded by #833.