mets: Use ISO8601 conformant timestamp for CREATEDATE, fix #357

[kba]

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[codecov-io]

Codecov Report

Merging #360 into master will decrease coverage by 0.03%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
- Coverage   85.32%   85.29%   -0.04%     
==========================================
  Files          30       30              
  Lines        1758     1761       +3     
  Branches      340      341       +1     
==========================================
+ Hits         1500     1502       +2     
  Misses        208      208              
- Partials       50       51       +1

Impacted Files	Coverage Δ
ocrd_models/ocrd_models/ocrd_mets.py	93.67% <75%> (-0.53%)	⬇️

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