- Before this change `InternalServer::build_redirect()` only URI-encoded the
  article path, ignoring the book name and/or the root location components of
  the URL.

- In order to be able to test this fix, corner_cases.zim was renamed to
  contain a couple of special URL symbols in its filename. The
  `create_corner_cases_zim_file` script was updated accordingly.

Testing of this functionality revealed that the query part containing +
symbols (as replacement for spaces in the parameter values) isn't
forwarded properly as the + symbols are URI-encoded (this is a bug on
the part of the `RequestContext::get_query()` the result of which
already contains URI-encoded +'s).

This change doesn't make much sense on its own - the real goal is to
prepare some ground for easier implementation of URI-encoding of the root
location.

This silly optimization in fact helps to avoid a somewhat more serious
waste of CPU cycles that would otherwise result in the next commit.

Now the root location is URI-encoded too.

In order to properly test this change the root location in the tests was
changed from "/ROOT" to "/ROOT#?" (or "/ROOT%23%3F" in URI-encoded form),
which is why this commit is so big.

The alleged bug seems rather an issue with httplib which seems to
URI-encode any + present in query parameters.