[1788] removed the default for 'filetype' from the spec file.

for optional items setting the default doesn't make sense, but we also have
test cases where syntax validation is skipped, so changing it to non-optional
will break existing test cases.  at least for now, it seems to make most sense
to keep it optional and define the default within the parser code.

src/bin/auth/auth.spec.pre.in

@@ -50,8 +50,7 @@
               },
               { "item_name": "filetype",
                 "item_type": "string",
-                "item_optional": true,
-                "item_default": "text"
+                "item_optional": true
               }]
             }
           }]

src/bin/auth/auth_config.cc

@@ -165,7 +165,6 @@ MemoryDatasourceConfig::build(ConstElementPtr config_value) {
             isc_throw(AuthConfigError, "Missing zone file for zone: "
                       << origin_txt);
         }
-        // XXX: we need to hardcode the default, see above.
         ConstElementPtr filetype = zone_config->get("filetype");
         const string filetype_txt = filetype ? filetype->stringValue() :
             "text";