Add tests to check Session id attr can be modified

Some modules, most notably Dancer2::Session::Cookie, require this to work.

See also #460.

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Document the fact the Session id attr can be modified

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Tidy tests using project .perltidyrc

Signed-off-by: Pedro Melo <melo@simplicidade.org>
reflect change
Merge branch 'pr/qa_for_session_id_is_rw'