Setting default Accept header #723

I wonder if we should consider having Unpoly set a default Accept header almost always.

I agree that up.render() almost always wants HTML.

up.request() often wants JSON (when people use it as a fetch() replacement), but sometimes HTML (when people do their own HTML fetching because they want to compose render steps differently than up.render()).

Unless someone is calling up.request directly, I'd presume they'd always want text/html.
Worst case, the default could still include / for fallback.

If we add a */* fallback it could be safe way to express "HTML if possible, but JSON is OK?", even for up.request()?

I think we should test e.g. how Rails handles a */* fallback. Will it still match a JSON view in cases like this?

def index
  @people = Person.all

  respond_to do |format|
    format.js
  end
end

Will it still render an implicit JSON view if there's no HTML view?

def index
  @people = Person.all
  # will it render index.json.jbuilder if there is no index.erb?
end