fix(schema): make aliases handle mongoose-lean-virtuals

Backport fix for #6069 and mongoosejs/mongoose-lean-virtuals#6

Author: vkarpov15

lib/schema.js

@@ -143,16 +143,20 @@ function aliasFields(schema) {
 
     if (alias) {
       if ('string' === typeof alias && alias.length > 0) {
-        if (schema.aliases[alias])
+        if (schema.aliases[alias]) {
           throw new Error('Duplicate alias, alias ' + alias + ' is used more than once');
-        else
+        } else {
           schema.aliases[alias] = prop;
+        }
 
         schema
           .virtual(alias)
           .get((function(p) {
             return function() {
-              return this.get(p);
+              if (typeof this.get === 'function') {
+                return this.get(p);
+              }
+              return this[p];
             };
           })(prop))
           .set((function(p) {