Add new Access-Control-Allow-Headers value into http response header

[lukas-vlcek]

Some http clients require Access-Control-Allow-Headers value to be set for preflight OPTIONS requests. I realized that for example jQuery needs this once I switched from 1.4.x to 1.5

[lukas-vlcek]

Add new Access-Control-Allow-Headers value into http response header, closed by 12a33e6

[kimchy]

Pushed the change.