Update Model.php

Fixed the case on the link to the book, to match the anchor in the page.
Merge pull request #3925 from davidyell/patch-1

Update Model.php
A micro optimization of h()

In most cases, the first argument of h() will be a string.
Merge pull request #3927 from chinpei215/master-h-opt

A micro optimization of h()
Remove flaky test that was of questionable value.
Merge branch 'master' into 2.6