Adding default values to fix correctness issue (#12286)

* Adding default values to fix correctness issue

* revoke default value for allowedOrigins

Author: juniwang

specification/signalr/resource-manager/Microsoft.SignalRService/preview/2020-07-01-preview/signalr.json

@@ -1343,7 +1343,8 @@
       "properties": {
         "defaultAction": {
           "$ref": "#/definitions/ACLAction",
-          "description": "Default action when no other rule matches"
+          "description": "Default action when no other rule matches",
+          "default": "Deny"
         },
         "publicNetwork": {
           "$ref": "#/definitions/NetworkACL",

specification/signalr/resource-manager/Microsoft.SignalRService/stable/2020-05-01/signalr.json

@@ -1340,7 +1340,8 @@
       "properties": {
         "defaultAction": {
           "$ref": "#/definitions/ACLAction",
-          "description": "Default action when no other rule matches"
+          "description": "Default action when no other rule matches",
+          "default": "Deny"
         },
         "publicNetwork": {
           "$ref": "#/definitions/NetworkACL",