fix: typecheck source param before parsing

[jesse-y]

Updates DOMParser.parseFromString to also check that the source provided is a string. To prevent sax from throwing undefined errors when calling indexOf on the source.

Adds unit tests to test/parse/simple.vows.js to cover this edge case.

Resolves #98

[brodybits]

Resolution looks OK to me, thanks.

We may rework the test cases in the future, as discussed in #71. But I think we may as well merge now to resolve the issue. Any comments from @karfau?

[jesse-y]

Yeah, I'd be fine with these tests being rewritten in another framework (I'm more familiar with Jest, so it's a +1 for me in regard to #72 ). Also happy to help on that front when some of the other housekeeping PRs (#102 #110 ) are in.

[karfau]

Sorry for the delay, go for it!

[karfau]

FYI: 0.9.0 will change this:
In case of XML parsing to be a fatalError, which causes DOMParser.parseFromString to throw in such a case.
(This change helps to avoid the situation, in which undefined is returned instead of a Document.)
In case of HTML parsing it will not be reported as any kind of error, since browsers also support passing arbitrary values, that are String()ified before being added to the body.