Parse and reformat datetimes only when defined via value-class pattern

[jmacdotorg]

This change updates four tests dealing with datetime attributes. The MF2 spec (as of July 2018) specifies that datetime attributes should be parsed and reformatted only when they are defined in the original document via value-class pattern. In every other instance, they are to be treated as literal strings, and passed through to the resulting JSON file unchanged.

The updates here deal specifically with datetimes not defined via value-class pattern, and which therefore should have JSON representations that match their appearance in their respective source HTML files.

[sknebel]

Thank you! Appears correct, so +1 to merge for me.