"Blank" string with space char only leading to RuntimeError when parsing as StringAttribute

I ran into this problem with awkward form input that gave blank fields as `" "`. This was (in my view incorrectly) raising a RuntimeError when parsing as StringAttribute, complaining that the value is not parseable. This commit fixes that problem and adds some tests for the String parser. The other parsers probably suffer of the same problem and might need some similar basic unit tests as well.

I also allowed myself to add (my own) simplecov gem for checking out test code coverage. It's in a separate commit, so if you don't like it feel free to ignore that one and just pull in the fixes/new specs.

Cheers