Sitemap tests added

As we lack tests for our code some tests for sitemap generator were implemented.
I believe, we have to use `spec` everywhere we can. https://docs.python.org/3/library/unittest.mock.html#autospeccing

It's a really wonderful feature which forbid to mock attributes that aren't present in some original object. I mean, if we don't use `spec` we can define any attribute in the mock. And if we use `spec` we can define only those attributes which are presented in the spec object.

It's really important, since if we remove, for example, the `abs_url` and run tests without `spec` the test will pass (the sitemap use `abs_url` and the mock has it).