Add better TypeError when assigning CPK

Raises a more readable error when assigning a singular ID to a
model with a composite primary key.

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -31,6 +31,7 @@ def primary_key_values_present? # :nodoc:
       # Sets the primary key column's value.
       def id=(value)
         if self.class.composite_primary_key?
+          raise TypeError, "Expected value matching #{self.class.primary_key.inspect}, got #{value.inspect}." unless value.is_a?(Enumerable)
           @primary_key.zip(value) { |attr, value| _write_attribute(attr, value) }
         else
           _write_attribute(@primary_key, value)

activerecord/test/cases/primary_keys_test.rb

@@ -426,9 +426,11 @@ def test_assigning_a_composite_primary_key
   def test_assigning_a_non_array_value_to_model_with_composite_primary_key_raises
     book = Cpk::Book.new
 
-    assert_raises(TypeError) do
+    error = assert_raises(TypeError) do
       book.id = 1
     end
+
+    assert_equal("Expected value matching [\"author_id\", \"number\"], got 1.", error.message)
   end
 
   def test_id_was_composite