Revision 1212318

Detect ASCII field types more rigorously,
and write their correct length when writing
TIFF tags.

This is based on bits from SANSELAN-51
and SANSELAN-12, however neither of those
is complete or correct by itself or even
when they're put together.

This resolves SANSELAN-12 and fixes part 1
of SANSELAN-51.

Jira issue keys: SANSELAN-12 , SANSELAN-51