[IMPR] Permit strings as exceptions for fixes

In other words, allow:

fixes['key'] = {

'exceptions': {
  'text-contains': 'skip this text',
},
'replacements': [
  ...
],

}

Previously, a string was interpreted as a sequence (list)
of characters, each being a pattern to search for. Which
would cause silent skipping of pages.

Change-Id: I0f4d01fcf96f6b22d3ddc5970138ef4db8259f85