Format function (%) doesn't support uppercase placeholders

[webberig]

It appears that the lessphp implementation of the format function doesn't support uppercase placeholders.

Here's an example:

@color: #333333;
@url: %("../img/sprite_icons_%D.svg", @color);
background-image: (@url);

The (expected) output using less.js:

background-image: "../img/sprite_icons_%23333333.svg";

The output using lessphp

background-image: "../img/sprite_icons_#333333.svg";

As you can see, there is no difference between %d and %D while the less documentation states the following:

Use uppercase placeholders if you need to escape special characters into their utf-8 escape codes. The function escapes all special characters except ()'~!. Space is encoded as %20. Lowercase placeholders leave special characters as they are.