Added rspec tests for layout_helper.rb

File is spec/helpers/layout_helper_spec.rb
https://www.pivotaltracker.com/story/show/82977718

it is a matter of personal preference, but in WebsiteOne, I am encouraging the use of `let` over instance variables, it allows you to do pretty neat tricks to keep tests clean (due to the lazy instantiation of `let` variables :grinning:), but it can also result in subtle bugs (due to lazy instantiation of `let` variables :frowning:). I won’t hold it against you if you choose to keep it this way though.