Fix strncmp size argument in WebDav/Server.php

[disconnect3d]

This commit fixes a bug where a strncmp($key, "HTTP_CONTENT", 11) is called with an off-by-one size parameter, as the "HTTP_CONTENT" string length (without the null byte) is 12 and not 11.

There is not much impact here - if someone sends a header ContenX where X can be any char other than 'T' the program will go to the default: branch and write out that this key is not supported:

                default: 
                    // any other unknown Content-* headers
                    $this->http_status("501 not implemented"); 
                    echo "The service does not support '$key'"; 
                    return;