fix: avoid URL-confusing default pathname in baseUrl

[platosha]

Fixes #517

When not set explicitly, the router constructor discovers baseUrl
using the <base href> value and initial document URL. However, only
the pathname from the discovered base URL was taken for the default
value of the baseUrl property. In case of initial pathname starting with
double slash //, such a value was then throwing Invalid URL error when
used as the URL constructor’s first argument.

This changes the default value discovery of baseUrl, so that it uses
an actual URL instead of only pathname to avoid throwing errors.