Stop file ext detection at query param boundaries

This commit improves Homebrew’s extension detector in `cask/lib/hbc/download_strategy.rb` a bit
so that it won’t cross individual URL query param boundaries any
longer:

```
def ext
  Pathname.new(@url).extname[/[^?&]+/]
end
```

Library/Homebrew/cask/lib/hbc/download_strategy.rb

@@ -154,7 +154,7 @@ module Hbc
     end
 
     def ext
-      Pathname.new(@url).extname[/[^?]+/]
+      Pathname.new(@url).extname[/[^?&]+/]
     end
   end