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 98.48%. Comparing base (876dd25) to head (29fbc25).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #409   +/-   ##
=========================================
  Coverage     98.48%   98.48%           
  Complexity      332      332           
=========================================
  Files            22       22           
  Lines          1059     1059           
=========================================
  Hits           1043     1043           
  Misses           16       16           

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