Stopped SparseFieldsetsMixin interpreting invalid fields parameter

[sliverc]

Fixes #519

Description of the Change

Stopped SparseFieldsetsMixin interpreting invalid fields query parameter

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #842 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #842      +/-   ##
==========================================
+ Coverage   97.76%   97.77%   +0.01%     
==========================================
  Files          58       58              
  Lines        3089     3104      +15     
==========================================
+ Hits         3020     3035      +15     
  Misses         69       69              

Impacted Files	Coverage Δ
example/tests/integration/test_sparse_fieldsets.py	100.00% <100.00%> (ø)

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[n2ygk]

Woops. Thanks for the fix!