title: Bourbon font-face outputting src surrounded by quotes
body: <p>So i have just recently upgraded to the latest bourbon (4.0.2 and sass (3.4.8). Since doing this, the <code>font-face</code> mixin outputs the src surrounded by double quotes, which is invalid and is breaking by fonts.</p>
  
  <p><code>@include font-face('Avenir', '../fonts/avenir/avenir-regular');</code></p>
  
  <p>Expected result (no double quotes around src):</p>
  
  <pre><code>@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');
  }
  </code></pre>
  
  <p>Output (double quotes around src):</p>
  
  <pre><code>@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')";
  
  }
  </code></pre>
  
  <p>The <code>font-face</code> mixin is as follows (see annotated line): </p>
  
  <pre><code>@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
      );
    }
  }
  </code></pre>
  
  <p>Is anyone else able to replicate this? </p>
  
  <p>Any ideas are really appreciated. </p>