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.