Fix request_fs returns raw buffer instead of string.

Since fs.readFile returns the raw buffer if no encoding is specified, the call to loadSVGFromString would fail. This PR fixes that, transforming the buffer into string (assuming 'utf-8') encoding. For other encodings, it may garble special characters.
Merge pull request #1135 from bernardofd/prJan2014

[Fix] loadSVGFromString receives raw buffer instead of string when loading SVG file from disk