Add failing test cases for issue #51
Field parsing will now fail if there is any unconsumed input

Before this change cassava (via attoparsec) would silently ignore
unconsumed input while parsing CSV fields.  If, for example, a CSV file
contained a field with "12.7" which was parsed into an Int, the ".7"
would be ignored (and skipped over).  This change triggers a field
parsing failure instead.

Closes issue #51.