Add tests for overwriting serializable_hash

This technique is useful when a user wants to generate
super complicated JSON.

test/usecases/user_defined_serializable_hash_test.rb

@@ -0,0 +1,32 @@
+require_relative '../test_helper'
+
+class UserDefinedSerializableHashTest < MiniTest::Test
+  class Foo
+    attr_accessor :id, :name
+
+    def initialize(id, name)
+      @id = id
+      @name = name
+    end
+  end
+
+  class FooResource
+    include Alba::Resource
+
+    def serializable_hash
+      grouped = @_object.group_by { |foo| foo.id.even? ? 'even' : 'odd' }
+      grouped.transform_values { |foos| foos.map { |foo| {name: foo.name} } }
+    end
+  end
+
+  def test_resource_class_with_overwriting_works
+    foo1 = Foo.new(1, 'name1')
+    foo2 = Foo.new(2, 'name2')
+    foo3 = Foo.new(3, 'name3')
+    foo4 = Foo.new(4, 'name4')
+    assert_equal(
+      '{"foos":{"odd":[{"name":"name1"},{"name":"name3"}],"even":[{"name":"name2"},{"name":"name4"}]}}',
+      FooResource.new([foo1, foo2, foo3, foo4]).serialize(with: proc { set key: :foos })
+    )
+  end
+end