assets: no content-length header present on asset response

[Daniel Hochman]

Description
Currently assets are returned from the handler without any content-length header. If the connection is closed prematurely for any reason, the browser won't know that it was premature, and will just assume that the file is complete.

If this is a Javascript file, the frontend just won't render at all and throw console errors only.

Expected Behavior
Content-Length headers are returned from the asset handling code in the mux.

Actual Behavior
No Content-length header is present.

Version
All

Other Context