Pass sourcesContent to sourceMap && add test

Ran into an issue with [`gulp-autoprefixer`](https://github.com/sindresorhus/gulp-autoprefixer/pull/3) that led to discovering that `gulp-less` doesn't properly pass `sourcesContent` to `sourceMap`. For example, if you have a `.less` file with an `@import`, you end up with both the imported file and importing file in `sources`, but a `null` in `sourcesContent` for the imported file. 

Per @floridoo, `sourcesContent` a required property. Seems to cause warnings/errors downstream when not passed properly.

This PR:

1. Adds a `sourcesContent` property to `sourceMap`
2. Recursively pushes `file.contents.toString()` from `sourceMap.sources` into `sourcesContent`
3. Adds a test for this functionality