Limit headers to something below 256kB

I'm not sure behavior is the same for every browser, but at least Chrome has a limit on the total header size (256kB). This causes ajax requests to fail without a clear error.
See issues https://github.com/barryvdh/laravel-debugbar/issues/41 and https://github.com/barryvdh/laravel-debugbar/issues/14
Some results for sizes are here: http://stackoverflow.com/questions/3326210/can-http-headers-be-too-big-for-browsers/3436155#3436155

This limits the max header to 244 kB, so it leaves about 10kB for additional headers.
Off course no data will be received at all, but is probably preferred to a failing ajax request.
Add error instead of empty array
Look for errors in header data
Merge pull request #50 from barryvdh/patch-1

Limit headers to something below 256kB