Fix default value of validateWCAG2Parms

[waitingcheung]

The function validateWCAG2Parms returns {"level":"AA", "size":"small"} for invalid parms. Since it is calling toUpperCase for level and toLowerCase for size, if either parms.level or parms.size is not of type string but evaluates to true, it will cause the following hard crash.

    size = (parms.size || "small").toLowerCase();
                                   ^

TypeError: (parms.size || "small").toLowerCase is not a function
    at validateWCAG2Parms (/path/tinycolor.js:1172:36)
    at Function.tinycolor.isReadable (/path/tinycolor.js:748:18)
    at Function.tinycolor.mostReadable (/path/tinycolor.js:792:19)