Fix for SourceComments including stdin instead of proper file path

[aphex]

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Resolves https://github.com/dlmanning/gulp-sass/issues/234

[Keats]

sorry for the wait, could you add a test that was previously failing for that to the testsuite?

[Snugug]

Yes, please add a test for this.

I'm also concerned that passing the file to Node Sass may result in unintended consequences of the raw file being compiled instead of the contents of the file stream. We already have a place in our codebase to transform stdin in to the main file's path (for sourcemaps); please refer to that,

[Snugug]

With the fix in #314, this is now failing. Can you please update?

[XhmikosR]

@xzyfer are you still interested in this? I rebased it, but unsure if you want to land it: https://github.com/dlmanning/gulp-sass/compare/master...XhmikosR:pr/286