Fix for download > 10mb issue with Content-Length: 0 being sent before

[jasonprogrammer]

Content-Length header with file size

[dom96]

IIRC you need to modify statusContent as doing this will fail for the asynchttpserver target (you can test by compiling with -d:useStdlib (again, if my memory is correct :)).

[jasonprogrammer]

I just tried compiling with -d:useStdlib, and you are correct (it hangs). :(

The problem is, calling statusContent calls send which ends up sending the undesired Content-Length: 0 header. It seems like there are multiple ways to fix this:

1. Use a when to detect when httpbeast/threads will be used, and if so, use the line in this PR.
2. Modify httpbeast to somehow not send the Content-Length: 0 when statusContent->send->send is called.

[dom96]

statusContent is implemented in jester, right? Why not use unsafeSend there (for httpbeast only)?