Set response content-length of zero to prevent client hangs

[zpalmtree]

If we have an empty body, the content length doesn't get set. This can cause hangs in some clients, for example, curl. A simple program to illustrate:

#include "httplib.h"

using namespace httplib;

int main()
{
    Server svr;

    svr.Get("/", [](const Request &req, Response &res)
    {
        res.status = 200;
    });

    svr.listen("127.0.0.1", 1234);
}

If we then perform curl http://127.0.0.1:1234 -v - We can see:

< HTTP/1.1 200 OK
* no chunk, no close, no size. Assume close to signal end
< 
* Closing connection 0

On my machine, this hangs for approximately 3 or so seconds.
If you then apply the patch in my PR, and try again, it should complete instantly:

< HTTP/1.1 200 OK
< Content-Length: 0
< 
* Connection #0 to host 127.0.0.1 left intact

Environment: Arch Linux 4.19.1, Curl 7.62.0

[yhirose]

@zpalmtree, thanks for your contribution! But it breaks the following unit test...

[ RUN      ] ServerTest.GetStreamedChunked
test.cc:836: Failure
Value of: res->body == "aaabbb"
  Actual: false
Expected: true

Could you try this new version? 5ad4311
It should fix your problem and pass all the unit test cases.

Thank you!

[zpalmtree]

My bad, I should have run the tests. Your fix works perfectly, thank you!

Oh - and thanks for great library - It's exactly what I was looking for - I took a while looking through all the libs, and they all seemed to heavy on dependencies, or very verbose - this is perfect ❤️