Merge pull request #607 from melissalinkert/rebased/dev_4_4/tiff-fixes

Minor defensive programming changes for TIFF-based formats (rebased onto dev_4_4)
Override getValue() in OriginalMetadataAnnotation

This prevents getValue() from returning null, as it previously delegated
to the implementation in XMLAnnotation, which returns XMLAnnotation's
internal 'value'.

Noticed by Stéphane Dallongeville.
merge dev_4_4 -D org -I include -E exclude: PR 617 (Override getValue() in OriginalMetadataAnnotation)