Add a spec for Dir#glob and escaped meta-characters

Hello,

This pull request simply adds a spec for `Dir#glob` dealing with an escaped comma. Using a backslash in front of a meta-character should normally escape it so if we escape a comma for instance, the glob shouldn't be split in two different parts when it contains curly braces.

We don't have to put this spec under the `platform_is_not(:windows)` block as commas are normally valid characters on NTFS systems (c.f. [here](http://stackoverflow.com/a/2272995/2663959)).

Have a nice day.
I realize this is quite late, but could the spec description be changed so that it matches more closely to what is being tested? I'm this particular case the spec doesn't test just any meta-character, instead it merely tests if you can glob for the literal "foo,bar" path by escaping the comma.

p.s. When doing so you probably want to rebase based on master, or just re-do the commit by hand.