Fix applySourceMap's aSourceMapPath parameter source content support

While improving gruntjs/grunt-contrib-concat#59 after @lydell's insight to use the third parameter of `SourceMapGenerator#applySourceMap` I noticed the `sourcesContent` was not being honored in my test cases. `sourcesContent` would be present, but the array was all `null` values. To fix that, I have added a small branch similar to that in the `this._mappings.forEach` block in `applySourceMap`. To test it, I modified the related test for the second and third parameter of `applySourceMap` to test if it is supporting  `sourcesContent`.