path: isAbsolute() should always return boolean

On Windows, path.isAbsolute() returns an empty string on failed cases.
This forces the return value to always be boolean.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>

lib/path.js

@@ -206,7 +206,7 @@ if (isWindows) {
   exports.isAbsolute = function(path) {
     var result = splitDeviceRe.exec(path),
         device = result[1] || '',
-        isUnc = device && device.charAt(1) !== ':';
+        isUnc = !!device && device.charAt(1) !== ':';
     // UNC paths are always absolute
     return !!result[2] || isUnc;
   };