Fix average on decimal and enum attributes

Follow up to #40351 and #41418.

This fixes `average` on decimal and enum (on integer in general)
attributes to not do `type.deserialize`.

The precision and the scale on the column might be lower than the
calculated result.

And also, mapping the calculated result to enum label is quite
meaningless, the mapping result is almost nil.

activerecord/lib/active_record/relation/calculations.rb

@@ -449,7 +449,12 @@ module ActiveRecord
         when "sum"
           type.deserialize(value || 0)
         when "average"
-          type.is_a?(Type::Integer) ? value&.to_d : type.deserialize(value)
+          case type.type
+          when :integer, :decimal
+            value&.to_d
+          else
+            type.deserialize(value)
+          end
         else # "minimum", "maximum"
           type.deserialize(value)
         end

activerecord/test/cases/calculations_test.rb

@@ -55,10 +55,10 @@ class CalculationsTest < ActiveRecord::TestCase
   end
 
   def test_should_return_integer_average_if_db_returns_such
-    ShipPart.delete_all
-    ShipPart.create!(id: 3, name: "foo")
-    value = ShipPart.average(:id)
-    assert_equal 3, value
+    value = Book.average(:status)
+
+    assert_equal 1.0, value
+    assert_instance_of BigDecimal, value
   end
 
   def test_should_return_float_average_if_db_returns_such
@@ -67,13 +67,21 @@ class CalculationsTest < ActiveRecord::TestCase
 
     assert_equal 37.5, value
     assert_instance_of Float, value
+
+    if current_adapter?(:PostgreSQLAdapter, :SQLite3Adapter)
+      NumericData.create!(temperature: "Infinity")
+      value = NumericData.average(:temperature)
+
+      assert_equal Float::INFINITY, value
+      assert_instance_of Float, value
+    end
   end
 
   def test_should_return_decimal_average_if_db_returns_such
-    NumericData.create!(bank_balance: 37.50)
+    NumericData.create!([{ bank_balance: 37.50 }, { bank_balance: 37.45 }])
     value = NumericData.average(:bank_balance)
 
-    assert_equal 37.50, value
+    assert_equal 37.475, value
     assert_instance_of BigDecimal, value
   end