When DownloadStrategyDetector.detect is given a second argument, and
that argument is not a symbol or an AbstractDownloadStrategy subclass,
it is silently ignored, and we fall back to guessing the strategy based
on the URL.
This means I can do
url 'http://foo.com/bar.tar.gz', :using => Class.new
and things will appear to work, even though I have clearly passed an
invalid value for :using.
A more useful behavior is to raise an exception for unknown strategy
specifications.
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