Allow expressions with a comma in switch case condition

The ES5 grammar specifies that the `case` keyword should be followed by an
expression (http://www.ecma-international.org/ecma-262/5.1/#sec-A.4) and that
an expression can consist of a comma-separated list of sub-expressions
(http://www.ecma-international.org/ecma-262/5.1/#sec-A.3), so the JSHint parser
should accomodate these commas.

I wasn't sure if I should first create a separate issue and then submit a PR, so I decided to just make a PR that includes a test case that fails without my fix, and passes with it. So that should demonstrate what would have been in the issue.