Ruby's URI.escape is now deprecated because different components of a
path require different escaping rules. However, we still want to
perform a similar escaping style, so let's do it ourselves with a
String#gsub call. We could use CGI.escape, but that encodes spaces as
plus signs and it's unclear whether that's a desirable change to make
here. Note that there are no negative consequences for encoding more
data than we want, since the server will decode it for us.
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