Do not ignore color stop positions for the first and last gradient colors

Input:

```css
.a {
    background: linear-gradient(to right, rgba(255,255,255,0) 30%,rgba(255,255,255,1) 100%);
}
```

Output (before fix):

```css
.a {
    background: -webkit-gradient(linear, left top, right top, from(rgba(255,255,255,0)), to(rgba(255,255,255,1)));
    background: linear-gradient(to right, rgba(255,255,255,0) 30%,rgba(255,255,255,1) 100%);
}
```

Output (after fix):

```css
.a {
    background: -webkit-gradient(linear, left top, right top, color-stop(30%, rgba(255,255,255,0)), to(rgba(255,255,255,1)));
    background: linear-gradient(to right, rgba(255,255,255,0) 30%,rgba(255,255,255,1) 100%);
}
```