Return raw value for numeric AVG calculation results

After this change `float`-typed database columns will return `Float`
when aggregated with `AVG` (as expected and consistent with the
behaviour of `MIN` and `MAX`).

We keep returning `BigDecimal` for non-numeric column types which
respond to `to_d`.

activerecord/lib/active_record/relation/calculations.rb

@@ -448,7 +448,11 @@ def type_cast_calculated_value(value, operation)
         when "sum"
           yield value || 0
         when "average"
-          value&.respond_to?(:to_d) ? value.to_d : value
+          if !value.is_a?(Numeric) && value&.respond_to?(:to_d)
+            value.to_d
+          else
+            value
+          end
         else # "minimum", "maximum"
           yield value
         end

activerecord/test/cases/calculations_test.rb

@@ -59,6 +59,14 @@ def test_should_return_integer_average_if_db_returns_such
     assert_equal 3, value
   end
 
+  def test_should_return_float_average_if_db_returns_such
+    NumericData.create!(temperature: 37.5)
+
+    value = NumericData.average(:temperature)
+    assert_instance_of Float, value
+    assert_equal 37.5, value
+  end
+
   def test_should_return_nil_as_average
     assert_nil NumericData.average(:bank_balance)
   end