Assume EnableSendFile On when unset

This corresponds to the documented behavior of Apache2(.2).
The "emulateSendFile" code is unfortunately kind of bad, as the 8k buffers
it uses doesn't work well with regular TCP MSS of 1492.

mod_xsendfile.c

@@ -354,7 +354,7 @@ static apr_status_t ap_xsendfile_output_filter(ap_filter_t *f, apr_bucket_brigad
     translated,
     APR_READ | APR_BINARY
 #if APR_HAS_SENDFILE
-    | (coreconf->enable_sendfile == ENABLE_SENDFILE_ON ?  APR_SENDFILE_ENABLED : 0)
+    | (coreconf->enable_sendfile != ENABLE_SENDFILE_OFF ? APR_SENDFILE_ENABLED : 0)
 #endif
     ,
     0,
@@ -373,7 +373,7 @@ static apr_status_t ap_xsendfile_output_filter(ap_filter_t *f, apr_bucket_brigad
     return HTTP_NOT_FOUND;
   }
 #if APR_HAS_SENDFILE && defined(_DEBUG)
-  if (coreconf->enable_sendfile != ENABLE_SENDFILE_ON) {
+  if (coreconf->enable_sendfile == ENABLE_SENDFILE_OFF) {
     ap_log_error(
       APLOG_MARK,
       APLOG_WARNING,