Don't directly mutate exclusion schema

Co-authored-by: mix irving <[email protected]>

spec/group/exclude-member.js

@@ -1,11 +1,17 @@
 const Validator = require('is-my-ssb-valid')
 const addMemberSchema = require('private-group-spec/group/add-member/v1/schema.json')
-const schema = require('private-group-spec/group/exclude-member/schema.json')
+const baseSchema = require('private-group-spec/group/exclude-member/schema.json')
 
 // we use the tribes2 exclude message type but with sigil links instead of URIs
-schema.definitions.feedId = addMemberSchema.definitions.feedId
-schema.definitions.groupId = addMemberSchema.definitions.cloakedMessageId
-schema.definitions.messageId = addMemberSchema.definitions.messageId
+const schema = {
+  ...baseSchema,
+  definitions: {
+    ...baseSchema.definitions
+    feedId: addMemberSchema.definitions.feedId
+    groupId: addMemberSchema.definitions.cloakedMessageId
+    messageId: addMemberSchema.definitions.messageId
+  }
+}
 
 module.exports = {
   isValid: Validator(schema)