use promote_rule not promote_type

[oxinabox]

Solves the ambiguity reported in https://discourse.julialang.org/t/having-trouble-implementating-a-tables-jl-row-table-when-using-badukgoweiqitools-dataframe-tbl-no-longer-works/63622
cc @xiaodaigh

I have a feeling that one is not mean to define promote_type directly.
(Will be making a PR to Julia to document that and then find out if i am wrong via Cunningham's law)

[xiaodaigh]

Thanks. I was mulling whether to report this to WeakRefStrings or DataFrames. But I wanted to push it for a couple of weeks.

[codecov]

Codecov Report

Merging #73 (ca27041) into main (ec317cc) will increase coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   81.48%   81.55%   +0.07%     
==========================================
  Files           3        3              
  Lines        1145     1144       -1     
==========================================
  Hits          933      933              
+ Misses        212      211       -1     

Impacted Files	Coverage Δ
src/inlinestrings.jl	90.87% <0.00%> (+0.17%)	⬆️

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[quinnj]

Thanks for the fix and clarification to Base! (failure on nightly is unrelated; I'll take a look at it separately)