Wrong output to escaped character

Input:
```css
.class { content: "\F10C" }
```
Output:
```css
.class { content: "\F10C" }
```
Seems works fine.

But wrong with lowercase.
Input:
```css
.class { content: "\f10C" }
```
Output:
```css
.class { content: "\F10C" }
```

I do not think it's true to change case of escaped character.