Do not set the Content-Length on the download response

Summary:
Seems related to:

• https://github.com/swoole/swoole-src/issues/4857
• https://github.com/laravel/octane/issues/568

Apparently swoole uses the chunked transfer encoding,
and according to the rfc we're not supposed to set Content-Length in
that case.

In any case with the Content-Length header swoole crashes, without
everything seems to work.

Differential Revision: https://git.kolab.org/D4187