Don't overwrite headers set by the framework

[brillout]

This line:

photon/packages/core/src/middlewares/devServer.ts

Line 19 in 0ae8795

response.headers.set("Cache-Control", "no-store");

Overwrites headers set by the framework/user. I can provide a reproduction.

[magne4000]

This behaviour has not been ported on #62, so once it's merged it should be fixed.