fix type coercion in bmerge

[ben-schwen]

Closes #6602

NB: I'm not happy that a user can get different messages, depending on the number of join conditions, but coercing both Dates to double when multiple column conditions exist, seems like the right decision.

Base does not encounter this problem since one join column can not be in multiple join conditions.

[github-actions]

Generated via commit 1d7a8c2

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 39 seconds
Installing different package versions	7 minutes and 26 seconds
Running and plotting the test cases	2 minutes and 13 seconds

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.60%. Comparing base (6a15f86) to head (1d7a8c2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6603   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files          79       79           
  Lines       14516    14520    +4     
=======================================
+ Hits        14314    14318    +4     
  Misses        202      202           

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[ben-schwen]

@MichaelChirico is it worth to make the if more specific to this corner case e.g.

(length(unique(icols))!=length(icols) || length(unique(xcols))!=length(xcols))

ensuring that either icols has a double condition or xcols has a double condition?