Solves User model validation bug

This solves the issue of the User model's pre('validate') method,
attempting to validate against a password that was not modified.

Adds the this.isModified('password') check to the condition.

modules/users/server/models/user.server.model.js

@@ -116,7 +116,7 @@ UserSchema.pre('save', function (next) {
  * Hook a pre validate method to test the local password
  */
 UserSchema.pre('validate', function (next) {
-  if (this.provider === 'local' && this.password) {
+  if (this.provider === 'local' && this.password && this.isModified('password')) {
     var result = owasp.test(this.password);
     if (result.errors.length) {
       var error = result.errors.join(' ');