Revert to 4.1 behavior for casting PG arrays

The user is able to pass PG string literals in 4.1, and have it
converted to an array. This is also possible in 4.2, but it would remain
in string form until saving and reloading, which breaks our
`attr = save.reload.attr` contract. I think we should deprecate this in
5.0, and only allow array input from user sources. However, this
currently constitutes a breaking change to public API that did not go
through a deprecation cycle.

activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb

@@ -34,6 +34,9 @@ def type_cast_from_database(value)
           end
 
           def type_cast_from_user(value)
+            if value.is_a?(::String)
+              value = parse_pg_array(value)
+            end
             type_cast_array(value, :type_cast_from_user)
           end
 

activerecord/test/cases/adapters/postgresql/array_test.rb

@@ -264,11 +264,26 @@ def test_datetime_with_timezone_awareness
 
   def test_assigning_non_array_value
     record = PgArray.new(tags: "not-an-array")
-    assert_equal "not-an-array", record.tags
-    e = assert_raises(ActiveRecord::StatementInvalid) do
-      record.save!
-    end
-    assert_instance_of PG::InvalidTextRepresentation, e.original_exception
+    assert_equal [], record.tags
+    assert_equal "not-an-array", record.tags_before_type_cast
+    assert record.save
+    assert_equal record.tags, record.reload.tags
+  end
+
+  def test_assigning_empty_string
+    record = PgArray.new(tags: "")
+    assert_equal [], record.tags
+    assert_equal "", record.tags_before_type_cast
+    assert record.save
+    assert_equal record.tags, record.reload.tags
+  end
+
+  def test_assigning_valid_pg_array_literal
+    record = PgArray.new(tags: "{1,2,3}")
+    assert_equal ["1", "2", "3"], record.tags
+    assert_equal "{1,2,3}", record.tags_before_type_cast
+    assert record.save
+    assert_equal record.tags, record.reload.tags
   end
 
   def test_uniqueness_validation