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.