Provide `HttpMessageConverter` for HAL-FORMS to fall back to HAL as content type

[odrotbohm]

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If the HAL-FORMS related processing of a RepresentationModel ends up without any templates added to the response, it would be nice if there was an easy way to fall back to HAL as media type of that response.

HAL Explorer currently only inspects the Content-Type for HAL-FORMS and disables all interaction elements not covered by templates. This causes issues for Spring Data REST's default resources.