Extracted spec date is not always a date

[tidoust]

Via exploration done in w3c/browser-specs#1268 (comment). The date property that Reffy sets is not a date for a couple of specs but rather:

• "to correct markup." for CSS 2.1
• "Table of Contents" for WOFF 1.0

The extraction logic should be adjusted. And there should be a check somewhere that makes sure that the date actually represents a date!