DateValue is now a bit more permissive with malformed dates

[piponazo]

In #1547 we got stricter about the ISO 8601 date formats. This PR should fix #2144 by being a bit more permissive with dates containing 0s in the month or day fields (check the notes from IPTC ).

[piponazo]

@jim-easterbrook let me know if this would be enough for your special case, or you think we should consider some additional cases.

[codecov]

Codecov Report

Merging #2146 (56c52be) into main (7aae68e) will increase coverage by 0.00%.
The diff coverage is 61.11%.

@@           Coverage Diff           @@
##             main    #2146   +/-   ##
=======================================
  Coverage   63.31%   63.31%           
=======================================
  Files          97       97           
  Lines       19127    19138   +11     
  Branches     9713     9718    +5     
=======================================
+ Hits        12111    12118    +7     
- Misses       4748     4751    +3     
- Partials     2268     2269    +1     

Impacted Files	Coverage Δ
src/value.cpp	72.61% <61.11%> (-0.16%)	⬇️

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[jim-easterbrook]

I'll wait for the nightly build incorporating this change before I test it. In #2144 I should have also tested setting dates with zeros in - this also fails with v0.27.5 and builds of 1.0.0 from a few days ago.

I'm not sure I like the setting out of range months and days to 0. I think it's libexiv2's job to read whatever is in the file and leave it to the application program to make sense of it.

(As for the title of this patch - it's not a malformed date, it's IPTC standard compliant!)

[jim-easterbrook]

Just realised mainDatesWithZeros is not merged yet, so won't be in the next nightly build. I've checked out the branch and built it myself. It's working as I'd expect it to so I'm happy with this PR.