Fix issue #200 - treat keywords str value as single keyword

[tomdonaldson]

This addresses a usability issue with regsearch() where if one mistakenly supplies a single str as the keywords value, too many results are returned.

[codecov]

Codecov Report

Merging #201 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   70.91%   70.92%   +0.01%     
==========================================
  Files          37       37              
  Lines        3912     3914       +2     
==========================================
+ Hits         2774     2776       +2     
  Misses       1138     1138

Impacted Files	Coverage Δ
pyvo/registry/regtap.py	44.18% <100%> (+0.87%)	⬆️

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[cbanek]

LGTM - I agree it doesn't make a lot of sense to treat a string like an array of chars. Thanks for picking this up!