Fix 454 calculate with dynamic default

[gushil]

Closes #
#454

Why is this the best possible solution? Were any other approaches considered?

I think this is the simplest solution.

What are the regression risks?

I don't think there is any.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[codecov-commenter]

Codecov Report

Merging #456 into master will decrease coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   83.11%   82.91%   -0.20%     
==========================================
  Files          25       25              
  Lines        3595     3553      -42     
  Branches      833      833              
==========================================
- Hits         2988     2946      -42     
  Misses        460      460              
  Partials      147      147              

Impacted Files	Coverage Δ
pyxform/xls2json.py	78.03% <100.00%> (+0.03%)	⬆️
pyxform/validators/error_cleaner.py	83.33% <0.00%> (-2.09%)	⬇️
pyxform/validators/updater.py	82.00% <0.00%> (-1.11%)	⬇️
pyxform/builder.py	76.73% <0.00%> (-0.46%)	⬇️
pyxform/validators/util.py	75.78% <0.00%> (-0.26%)	⬇️
pyxform/survey.py	91.34% <0.00%> (-0.18%)	⬇️
pyxform/xform2json.py	79.33% <0.00%> (-0.09%)	⬇️
pyxform/survey_element.py	90.08% <0.00%> (-0.09%)	⬇️

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[MartijnR]

Thanks! Looks good to me judging from the tests.