It turned out that my patch was incomplete. Fixing the bug in
rng-uri-file-name-1 revealed another bug in rng-uri-unescape-unibyte.
The anonymous function passed to replace-regexp-in-string returned a
number (e.g. 32 for space) but replace-regexp-in-string expected a
string as the return value.

I therefore added a call to "string" to convert the number back to a
string (e.g. " ").

With this fix I was finally able to use a RELAX-NG schema file in a
path containing spaces.

I made the same fix in rng-uri-unescape-unibyte-match because it was
also obviously wrong (regexp-quote expects a string). However,
it would be good if someone more familiar with this code (and Lisp in
general) could check all the other occurrences of
replace-regexp-in-string to make sure they do not contain the same bug.

Find attached the expanded patch.