Upsert with "Prefer: resolution=merge-duplicates" on specified columns (#1327) (#1432)

Allow query param 'on_conflict=key1,key2,...' to upsert with explicit columns with unique
constraint.

test/fixtures/data.sql

@@ -344,6 +344,12 @@ INSERT INTO employees VALUES
 TRUNCATE TABLE tiobe_pls CASCADE;
 INSERT INTO tiobe_pls VALUES ('Java', 1), ('C', 2), ('Python', 4);
 
+TRUNCATE TABLE single_unique CASCADE;
+INSERT INTO single_unique (unique_key, value) VALUES (1, 'A');
+
+TRUNCATE TABLE compound_unique CASCADE;
+INSERT INTO compound_unique (key1, key2, value) VALUES (1, 1, 'A');
+
 TRUNCATE TABLE only_pk CASCADE;
 INSERT INTO only_pk VALUES (1), (2);
 

test/fixtures/privileges.sql

@@ -66,6 +66,8 @@ GRANT ALL ON TABLE
     , perf_articles
     , employees
     , tiobe_pls
+    , single_unique
+    , compound_unique
     , only_pk
     , family_tree
     , managers

test/fixtures/schema.sql

@@ -1379,6 +1379,18 @@ create table test.tiobe_pls(
   rank smallint
 );
 
+create table test.single_unique(
+  unique_key integer unique not null,
+  value text
+);
+
+create table test.compound_unique(
+  key1 integer not null,
+  key2 integer not null,
+  value text,
+  unique(key1, key2)
+);
+
 create table test.family_tree (
   id text not null primary key,
   name text not null,