Add OC_Response::setContentLengthHeader() for Apache PHP SAPI workaround.

Do not send Content-Length headers with a value larger than PHP_INT_MAX
(2147483647) on Apache PHP SAPI 32-bit. PHP will eat them and send 2147483647
instead.

When X-Sendfile is enabled, Apache will send a correct Content-Length header,
even for files larger than 2147483647 bytes. When X-Sendfile is not enabled,
ownCloud will not send a Content-Length header. This prevents progress bars
from working, but allows the actual transfer to work properly.