Remove -pie prefix from background()

[saschagehlich]

Input:

a {
  @include background(linear-gradient(to right, white, black))
}

Output:

// ...
-pie-background: -pie-linear-gradient(to right, white, black)
// ...

Pie does not recognize linear-gradient with a -pie- prefix.

[Igosuki]

I checked PIE and indeed linear-gradient has no prefix