Fix error encountered while converting multi-language XLSForms to JSON

[DavisRayM]

Closes #513

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

The simplest option removes objects that can't be converted to JSON.

What are the regression risks?

• N/A

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:

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

[codecov-io]

Codecov Report

Merging #514 (f2515d3) into master (1c4a2ed) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   83.81%   83.86%   +0.04%     
==========================================
  Files          25       25              
  Lines        3694     3699       +5     
  Branches      860      862       +2     
==========================================
+ Hits         3096     3102       +6     
+ Misses        453      452       -1     
  Partials      145      145              

Impacted Files	Coverage Δ
pyxform/survey_element.py	95.06% <100.00%> (+0.52%)	:arrow_up:

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