Add :form and :multipart formats

[cllns]

These are commonly used for web apps so we should have first-class support for them.

Related: #439

Also, I made Hanami::Action::Mime::TYPES into @api public (from private). The values are already used in apps so it's effectively already public API, so we might as well show them in the API docs. We already link to the constant in the guides, so in the future that could be a link to API docs (once those are working properly).

[afomera]

I tried this branch out with my application, and was able to resolve an issue I had where my Hanami apps had to define the form format for basic form submissions. 🙌

[timriley]

This is an unequivocally good first step towards improving this stuff. Thanks @cllns! Bringing this in as part of our improvements to format handling for 2.3.

[timriley]

@cllns Now that I think about it, and after putting together #485, where it is clear that users can add formats with multiple associated MIME types, I wonder if it would make more sense to group the "application/x-www-form-urlencoded" and "multipart/form-data" types under the :html format? There all part of the capabilities that are exposed from HTML, after all?

I'm thinking about the scenario of a user doing config.formats.accept :html in one of their actions. In this case, their action would no longer accept form posts with a Content-Type of application/x-www-form-urlencoded. This means that their actions would reject standard form submissions! (As well as form submissions containing files).

To me, "having my actions accept form submissions" feels like a bog basic aspect of "supporting HTML". In this way, I think that users should not have to remember to add :form, :multipart to the list whenever they do accept :html in their actions.

If we rejigged this TYPES constant to allow multiple MIME type codes to correspond to a format (which the user can already do via their manual config.formats.register), then I think suddenly everything would just work.

I want to get this stuff right (as much as possible, at least) before we release beta2, so I think I'm going to have a play in a test app and explore this idea, but I wanted to write it here first in case you had any feedback in the meantime :)

[timriley]

Update: I'm taking a slightly different approach. I hope to have some code to share in the next day or two.

The approach is based on storing mime types and content types separately for formats, which allows us to do this:

          html: Format.new(
            name: :html,
            mime_type: "text/html",
            content_types: ["application/x-www-form-urlencoded", "multipart/form-data"]
          )

[timriley]

OK, these entries are now removed as of #485.

To ensure these content types are accepted for HTML formats, we have the following default format registered:

html: Format.new(
  name: :html,
  media_type: "text/html",
  content_types: ["application/x-www-form-urlencoded", "multipart/form-data"]
)

What this means is this: if :html is configured as an accepted format, then these content types will be permitted, and the response's content type is still set to text/html by default.