Bourbon font-face outputting src surrounded by quotes

Question

So i have just recently upgraded to the latest bourbon (4.0.2 and sass (3.4.8). Since doing this, the font-face mixin outputs the src surrounded by double quotes, which is invalid and is breaking by fonts.

@include font-face('Avenir', '../fonts/avenir/avenir-regular');

Expected result (no double quotes around src):

@font-face {
  font-family: "Avenir";
  font-weight: normal;
  font-style: none;
  src: url('../fonts/avenir/avenir-regular.eot') format('embedded-opentype');
  src: url('../fonts/avenir/avenir-regular.eot?#iefix') format('embedded-opentype'), url('../fonts/avenir/avenir-regular.woff') format('woff'), url('../fonts/avenir/avenir-regular.ttf') format('ttf'), url('../fonts/avenir/avenir-regular.svg#avenir') format('svg');
}

Output (double quotes around src):

@font-face {
  font-family: "Avenir";
  font-weight: normal;
  font-style: none;
  src: "url('../fonts/avenir/avenir-regular.eot') format('embedded-opentype')";
  src: "url('../fonts/avenir/avenir-regular.eot?#iefix') format('embedded-opentype')", "url('../fonts/avenir/avenir-regular.woff') format('woff')", "url('../fonts/avenir/avenir-regular.ttf') format('ttf')", "url('../fonts/avenir/avenir-regular.svg#avenir') format('svg')";

}

The font-face mixin is as follows (see annotated line):

@mixin font-face(
  $font-family,
  $file-path,
  $weight: normal,
  $style: normal,
  $asset-pipeline: $asset-pipeline,
  $file-formats: eot woff ttf svg) {

  $font-url-prefix: font-url-prefixer($asset-pipeline);

  @font-face {
    font-family: $font-family;
    font-weight: $weight;
    font-style: $style;

    @if contains($file-formats, eot) {
      // THIS IS THE TROUBLE LINE
      src: "#{$font-url-prefix}('#{$file-path}.eot') format('embedded-opentype')";
    }

    src: font-source-declaration(
      $font-family,
      $file-path,
      $asset-pipeline,
      $file-formats,
      $font-url-prefix
    );
  }
}

Is anyone else able to replicate this?

Any ideas are really appreciated.