MDL-39378 Improve s() performance.

These changes give about a 10% speed-up in this function. The significant changes are:
1. Simplify the if logic to remove unnecssary cases.
2. Dont pass default argument values to htmlspecialchars, just using the
   defaults is faster.
3. I can confirm that /i regex is faster than the equivalent regex without the i.

I also added more unit tests to test the edge cases.