Raise when given an invalid download strategy spec

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.