Allow to return primitive data structures in fun

[tank-bohr]

What I've done

bookish_spork:stub_request(fun(Request) ->
    case bookish_spork_request:uri(Request) of
        "/bookish/spork" ->
            [200, [], <<"Hello">>];
        "/admin/sporks" ->
            [403, [], <<"It is not possible here">>]
    end
end)

Actual result

• It doesn't work
• We have to use clumsy syntax
• too many bookish_spork

Expected result

• Neat concise clojure-like syntax is allowed

Environment

• bookish_spork version is 0.2.5