In Safari a trailing '#' is appended to URIs extracted from web
documents and PDFs. For URLs this makes no difference as the Hypothesis
search API ignores the fragment identifier. However a URN with a
trailing '#' is considered different from a URN without a trailing '#'.
Since the URI for PDFs is now a URN of the form 'urn:x-pdf:...', when
viewing a PDF in Safari annotations made in other browsers were not
fetched and vice-versa.

The trailing '#' is caused by a bug in the `URL.hash` setter in Safari.

According to section 6.3 of the URL spec [1], setting URL.hash to an empty
string should _remove_ the fragment identifier. In Safari however, it
sets the fragment identifier to an empty string. The result is that when
the URL is serialized, the output is eg. 'urn:x-pdf:aabb#' instead of
'urn:x-pdf:aabb'.

Upstream bug: https://bugs.webkit.org/show_bug.cgi?id=158869
Fixes #3471

[1] https://url.spec.whatwg.org/#urlutils-members