Fixing typos/method calls in grabValueFrom

grabValueFrom calling wrong methods and incorrect comments regarding
types.
- filter() returns Crawler
- Crawler does not have childNodes
- children() returns DOMElement objects
Adding test for grabValueFrom with a select field

Testing grabbing a value from a select field using grabValueFrom (also
added intended test for textarea which was missing)
Merge pull request #1512 from zbateson/fix-grabValueFrom-typo

Fixing typos/method calls in grabValueFrom