Fix multi-level expand Param subobject

[philippjfr]

Fixes #630

[codecov]

Codecov Report

Merging #965 into master will decrease coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
- Coverage   85.33%   85.33%   -0.01%     
==========================================
  Files          96       96              
  Lines       10747    10752       +5     
==========================================
+ Hits         9171     9175       +4     
- Misses       1576     1577       +1

Impacted Files	Coverage Δ
panel/param.py	90.53% <100%> (-0.05%)	⬇️
panel/util.py	92.24% <85.71%> (-0.38%)	⬇️

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