Modify ViewResolver auto-ContenType mechanism to make use of template resolution

[danielfernandez]

The automatic content type resolution mechanism introduced in #138 should be modified in Thymeleaf 3.1 after the implementation of thymeleaf/thymeleaf#608, so that it makes use not of file extensions in view names, but rather (or additionally, or alternatively) of the separate template resolution phase that should be made possible in 3.1

[danielfernandez]

Decline reason: The ContentType autocomputation mechanism should be based on the HTTP request and not the view name or the resolved template name.

Using the request (file extension in path and Accept header, in this order) is precisely what the Spring 5 content negotiation mechanism does.

Template Mode should be completely independent from this and the responsibility of the template resolvers. No template mode should be forced from the View/ViewResolver so that, for example, an .html page can be processed in TEXT if needed.