Quote whitespace-only default values

Parameters that accept whitespace-only values need to have their default
values quoted if they contain only whitespace characters. In 2.2 the
qlafilter is the only module that did not do this.

server/modules/filter/qlafilter/qlafilter.cc

@@ -306,7 +306,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
             {
                 PARAM_NEWLINE,
                 MXS_MODULE_PARAM_QUOTEDSTRING,
-                " ",
+                "\" \"",
                 MXS_MODULE_OPT_NONE
             },
             {