Source with inline sourcemaps

Hi,

I'm getting a different behavior with the same sourcemap either provided as external file or inline.
The external works well and provides an absolute path, the inline one just gives me the raw declaration.

Shouldn't [this line](https://github.com/evanw/node-source-map-support/blob/636b1b28e42c7cd3aa7fdfd9949ecd5bb9aef701/source-map-support.js#L148) be `sourceMap.url || position.source` ? At least it works consistently on my test.