Making proxy to be compliant with Sinatra 1.4.2 release

This change in Sinatra 1.4.2

https://github.com/sinatra/sinatra/pull/626/files

introduces new class AcceptEntry which is used for comparison of the accept
flag. Since we use request.accept.include? it does not work anymore on my
system:

(rdb:3) pp request.accept
[application/json]

(rdb:3) pp request.accept.include?("application/json")
false

(rdb:3) pp request.accept[0].class
Sinatra::Request::AcceptEntry

This patch changes all places where we use this comparison and changes it to
the standard way. This should be compatible across old Sinatra versions, but
please confirm this explicitly.