bug#23140: url-package-name and url-package-version are ignored by url-retrieve and url-retrieve-synchronously

[Clément Pit--Claudel]

url-package-name and url-package-version are only used in the initializer of 
the url-user-agent, which is computed when url.el is loaded; this makes these 
two variables essentially useless; in particular, one can't let-bind them 
around a call to url-retrieve.

The fix is probably to make url-user-agent accept nil, and make the 
url-http-user-agent-string function generate the default string every time it 
is called, based on the current values of url-package-name and 
url-package-version.

I have some code to do this, but I'd just like to make sure that it's the right 
approach before I propose a patch.

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