add reusable _update_set_values method, consistent with _insert_values

[kraih]

As discussed on IRC, this patch adds a reusable _update_set_values method that can be reused by SQL::Abstract extensions, just like _insert_values. The specific use case i have in mind for this will be PostgreSQL upserts, which rely on INSERT ... ON CONFLICT DO UPDATE SET ... queries.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.07%.
The diff coverage is 93.33%.

@@           Coverage Diff           @@
##           master   #12      +/-   ##
=======================================
+ Coverage   82.93%   83%   +0.07%     
=======================================
  Files           3     3              
  Lines         920   924       +4     
  Branches      189   189              
=======================================
+ Hits          763   767       +4     
  Misses        107   107              
  Partials       50    50

Impacted Files	Coverage Δ
lib/SQL/Abstract.pm	82.2% <93.33%> (+0.11%)	⬆️

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