Allow specs to pass values to request env

[drewish]

By adding a let(:env) to a spec you can provide additional values to add to the Rack environment hash (or headers in Rails 3 and 4). This is useful for providing additional headers that you don't want to explicitly document:

RSpec.describe 'accounts', type: :request, capture_examples: true do
  let(:env) { {'HTTP_AUTHORIZATION' => "Basic #{Base64.strict_encode64(api_key)}"} }

  # ...
end

Fixes #23

[drewish]

This still needs some tests.