Fixed usage of bson-objectid

no-issue

Calling ObjectId doesn't return a string a but an ObjectId object.
Whilst this object is cast to a string via the toJSON and toString
methods, this is not enough for MySQL. Instead we should explicitly cast
this to a string ourselves and the application level.

ghost/members-api/lib/repositories/member.js

@@ -400,7 +400,7 @@ module.exports = class MemberRepository {
                 return {
                     member_id: id,
                     label_id: data.meta.label.id,
-                    id: ObjectId()
+                    id: ObjectId().toHexString()
                 };
             });