Custom must_be_json minitest assertion / expectation for use with rack-test

gistfile1.rb

module MiniTest::Assertions
  def assert_content_type_is_json(response)
    content_type = response.headers['Content-Type']
    assert_equal 'application/json;charset=utf-8', content_type
  end
end

module MiniTest::Expectations
  infect_an_assertion :assert_content_type_is_json, :must_be_json, :unary
end

# Assertion
assert_content_type_is_json expected, last_response

# Expectation
last_response.must_be_json