Add form_with to unify form_tag/form_for.

`form_tag` and `form_for` serve very similar use cases. This
PR unifies that usage such that `form_with` can output just
the opening form tag akin to `form_tag` and can just work with
a url, for instance.

`form_with` by default doesn't attach class or id to the form —
removing them on fields is moved out to a default revisiting PR later.

Ported over old tests where applicable to ensure maximum coverage,
but left some commented out because they don't yet apply (e.g.
`fields_for` later being replaced by `fields`).

[ Kasper Timm Hansen & Marek Kirejczyk ]

actionview/lib/action_view/helpers/form_helper.rb

@@ -474,6 +474,71 @@ def apply_form_for_options!(record, object, options) #:nodoc:
  end
  private :apply_form_for_options!
 
+ # Passing model: @post will 1) set scope: :post, 2) set url: url_for(@post)
+ #
+ # form_with(model: @post) do |form|
+ # form.text_field :title # Will reference @post.title as normal
+ # form.text_area :description, "Overwrite @post.description if present, if not, it will still work"
+ #
+ # form.submit
+ # end
+ #
+ # form_with(scope: :post, url: posts_path) do |form|
+ # form.text_field :title # post[title]
+ # form.text_area :description, "Overwrite @post.description or ignore if it's not present"
+ #
+ # form.submit
+ # end
+ # # =>
+ # <form action="/posts" accept-charset="utf-8" method="post">
+ # <input name="utf8" type="hidden" value="✓">
+ # <input type="hidden" name="authenticity_token" value="...">
+ #
+ # <input type="text" name="post[title]">
+ # <textarea name="post[description]"></textarea>
+ #
+ # <input type="submit" name="commit" value="Create Post" data-disable-with="Create Post">
+ # </form>
+ #
+ # form_with(url: different_path, class: 'something', id: 'specific') do |form|
+ # form.text_field :title, 'This is the value of the title'
+ #
+ # form.text_area :description, class: 'No value has been supplied here'
+ #
+ # form.fields(:permission) do |fields|
+ # # on/off instead of positional parameters for setting values
+ # fields.check_box :admin, on: 'yes', off: 'no'
+ # end
+ #
+ # form.select :category, Post::CATEGORIES, blank: 'None'
+ # form.select :author_id, Person.all.collect { |p| [ p.name, p.id ] }, blank: 'Pick someone'
+ #
+ # form.submit
+ # end
+ def form_with(model: nil, scope: nil, url: nil, format: nil, html: {}, **options)
+ if model
+ url ||= polymorphic_path(model, format: format)
+
+ model = model.last if model.is_a?(Array)
+ scope ||= model_name_from_record_or_class(model).param_key
+ end
+
+ html_options = html.merge(options.except(:index, :include_id, :builder))
+ html_options[:method] ||= :patch if model.respond_to?(:persisted?) && model.persisted?
+
+ if block_given?
+ builder = instantiate_builder(scope, model, options)
+ output = capture(builder, &Proc.new)
+ html_options[:multipart] ||= builder.multipart?
+
+ html_options = html_options_for_form(url || {}, html_options)
+ form_tag_with_body(html_options, output)
+ else
+ html_options = html_options_for_form(url || {}, html_options)
+ form_tag_html(html_options)
+ end
+ end
+
  # Creates a scope around a specific model object like form_for, but
  # doesn't create the form tags themselves. This makes fields_for suitable
  # for specifying additional model objects in the same form.
@@ -1183,7 +1248,7 @@ def instantiate_builder(record_name, record_object, options)
  object_name = record_name
  else
  object = record_name
- object_name = model_name_from_record_or_class(object).param_key
+ object_name = model_name_from_record_or_class(object).param_key if object
  end
 
  builder = options[:builder] || default_form_builder_class