cast non-str column names to str when regex=True

[cosmicBboy]

fixes #521

[codecov]

Codecov Report

Merging #551 (e2d3f61) into dev (810a0ab) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #551      +/-   ##
==========================================
- Coverage   99.35%   99.31%   -0.04%     
==========================================
  Files          21       21              
  Lines        2632     2630       -2     
==========================================
- Hits         2615     2612       -3     
- Misses         17       18       +1     

Impacted Files	Coverage Δ
pandera/schema_components.py	99.51% <ø> (-0.49%)	⬇️

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