default_format not applied if content-type not supplied

Follow-up on old issue https://github.com/ruby-grape/grape/issues/407

format and default_format are set to :json yet when content-type is not supplied it doesn't parse correctly:

curl --include \
     --request POST \
   --data-binary "{
    \"email\": \"test@test.com\",
    \"password\": \"password\"
}" \
'localhost:3000/api/v1/login'

results in:

#<Hashie::Mash {
    "email": "test@test.com",
    "password": "password"
}=nil>

The test doesn't handle when content type isn't supplied at all, it's given as a blank string:

    it 'parses data in default format' do
      subject.post '/data' do
        { x: params[:x] }
      end
      post '/data', '{"x":42}', 'CONTENT_TYPE' => ''
      expect(last_response.status).to eq(201)
      expect(last_response.body).to eq('{"x":42}')
    end

When I remove that content type and post without it, it breaks without even making it to #read_rack_input and test fails.

These are both false in #read_body_input so it returns here:

(!request.form_data? || !request.media_type)

content_type/media_type default to "application/x-www-form-urlencoded" when content-type isn't supplied. And since request.media_type is "application/x-www-form-urlencoded" instead of nil, it wouldn't use the default_format here either even if we made it to this point:

from grape/lib/grape/middleware/formatter.rb #read_rack_input

fmt = mime_types[request.media_type] if request.media_type
fmt ||= options[:default_format]

Start by reproducing the curl request without a Content-Type header and compare it with the existing default-format test. Read grape/lib/grape/middleware/formatter.rb, especially #read_body_input and #read_rack_input, to trace the request media type and default format handling. Done means JSON data without a Content-Type is parsed using default_format.