Use StringColumn for decimal type by default

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

Related to yiisoft/db#1125

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (f3d04fc) to head (1bd9feb).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #472   +/-   ##
=========================================
  Coverage     96.68%   96.68%           
  Complexity      412      412           
=========================================
  Files            66       66           
  Lines          1207     1207           
=========================================
  Hits           1167     1167           
  Misses           40       40           

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