fix: external css properly handle html5-compliant syntax

[FRSgit]

vue-jest fails when using proper html5 syntax when including external css files.

When I import external CSS in a following way (that's something recommended by vue-loader and only way handled properly by vetur - they do not support highlighting of self-closing style tags):

<style src="./_bootstrap.module.scss" module="$styleBootstrap"></style>

vue-jest fails - it treats this style block as there would be no src attribute at all!

Fixed that case (added content.trim() - that's because vue/component-compiler for <style></style> returns bunch of spaces - something like: " ").

[lmiller1990]

Great, will need rebase (conflicts).

[FRSgit]

Yea, sure, didn't want to ship these two things (this one & #286) in a single PR, so the conflicts were inevitable here.
It's 2am for me now, will do this tomorrow 😄

[lmiller1990]

No rush, I tend to releases on fri/sat anyway, so I can test it out a bit!

[FRSgit]

@lmiller1990 done!

[lmiller1990]

I will release early next week.