fix non-string sourceMap option, fixes #337

sass.js

@@ -43,6 +43,7 @@ var prepareOptions = function (options) {
   var error;
   var stats;
   var sourceComments;
+  var sourceMap;
 
   options = options || {};
   success = options.success;
@@ -53,6 +54,11 @@ var prepareOptions = function (options) {
   if (options.sourceMap && !sourceComments) {
     sourceComments = 'map';
   }
+  sourceMap = options.sourceMap;
+  if (typeof sourceMap !== 'string' && sourceComments === 'map') {
+    sourceMap = '';
+  }
+
   prepareStats(options, stats);
 
   return {
@@ -64,7 +70,7 @@ var prepareOptions = function (options) {
     style: SASS_OUTPUT_STYLE[options.output_style || options.outputStyle] || 0,
     comments: SASS_SOURCE_COMMENTS[sourceComments] || 0,
     stats: stats,
-    sourceMap: options.sourceMap,
+    sourceMap: sourceMap,
     precision: parseInt(options.precision) || 5,
     success: function onSuccess(css, sourceMap) {
       finishStats(stats, sourceMap);