Fix strip_quotes() to handle empty strings
strip_quotes() assumed its parameter is at least one character long, and errored out on an empty string.
It also converted a string consisting of a single quote to an empty string because that single quote had a quote as first and last char.
This commit fixes these two bugs.
Also rewrite TestStripQuotes to use tests[], and add some test for an empty string, a one-char path (just a slash) and a single quote.
I propose this patch for 2.11..master.