Fix isDataURI returning false for perfectly valid strings, more info can be found here: #2121

I've added an additional test case with a MIME type that has an underscore in its name and added the _ (underscore) to the RegExp to match for these cases.

This fixes #2121

(on a side note here: I feel like isDataURI could or maybe even should be rewritten to internally use isMimeType and isBase64 -> that way you would get rid of some code duplication - but that is a completely different topic than this quick bug fix).

Checklist

• PR contains only changes related; no stray files, etc.
• [ ] README updated (where applicable)
• Tests written (where applicable)