RSpec matcher for parsing `response.headers['Content-Type']`

have_content_type.rb

RSpec::Matchers.define :have_content_type do |content_type|
  CONTENT_HEADER_MATCHER = /^(.*?)(?:; charset=(.*))?$/

  chain :with_charset do |charset|
    @charset = charset
  end

  match do |response|
    _, content, charset = *content_type_header.match(CONTENT_HEADER_MATCHER).to_a

    if @charset
      @charset == charset && content == content_type
    else
      content == content_type
    end
  end

  failure_message_for_should do |response|
    if @charset
      "Content type #{content_type_header.inspect} should match #{content_type.inspect} with charset #{@charset}"
    else
      "Content type #{content_type_header.inspect} should match #{content_type.inspect}"
    end
  end

  failure_message_for_should_not do |model|
    if @charset
      "Content type #{content_type_header.inspect} should not match #{content_type.inspect} with charset #{@charset}"
    else
      "Content type #{content_type_header.inspect} should not match #{content_type.inspect}"
    end
  end

  def content_type_header
    response.headers['Content-Type']
  end
end

my_controller_spec.rb

require 'spec_helper'

class MyController < ApplicationController
  respond_to :json, :only => :json

  def json
    respond_with(:json => 'rocks')
  end
end

MyApp::Application.routes.draw do
  match '/my_controller/json' => 'api_actions#json'
end

describe MyController do
  describe 'GET #json' do
    it 'responds with application/json' do
      get :json, :format => :json
      response.should have_content_type('application/json')
    end
  end
end