#361: Fix for quoted data URIs getting prepended with path

[asolove]

In the browser environment, quoted data URIs are prepended with the import path.

The issue superficially appeared to be fixed because the tests pass, but that was because the codepath with the bug is only run in the browser environment.

This ticket fixes the problem by correctly pattern-matching for valid data URIs, which do not require a slash.

[asolove]

This actually created a much larger browser-only bug, where all strings are treated as absolute URLs. This happened because I removed the slash after the optional group, so the regex now always matches all strings:

/^(?:https?:\/\/|file:\/\/|data:)?/ // matches a string starting with anything

The question is what to do in the case where the URL starts with a slash. If urls starting with a slash are treated as absolute, we can simply change the regex to:

/^(?:https?:\/\/|file:\/\/|data:|\/)/ // matches a string with http(s), file, data, or a slash

But the line inside the conditional, which only runs if the regex doesn't match, (and therefore the url is being treated as relative) seems to expect that urls starting with slash might come to it:

val.value = paths[0] + (val.value.charAt(0) === '/' ? val.value.slice(1) : val.value);

In which case the regex should be changed to non-optional but not include the slash option:

/^(?:https?:\/\/|file:\/\/|data:)/ // matches a string with http(s), file, or data only.