Cater to download strategies not yielding a file

Closes Homebrew/homebrew#22580.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula.rb

@@ -611,7 +611,7 @@ class Formula
 
   def stage
     fetched = fetch
-    verify_download_integrity(fetched) if fetched.file?
+    verify_download_integrity(fetched) if fetched.respond_to?(:file?) and fetched.file?
     mktemp do
       downloader.stage
       # Set path after the downloader changes the working folder.

Library/Homebrew/resource.rb

@@ -25,7 +25,7 @@ class Resource
   # A target or a block must be given, but not both
   def stage(target=nil)
     fetched = fetch
-    verify_download_integrity(fetched) if fetched.file?
+    verify_download_integrity(fetched) if fetched.respond_to?(:file?) and fetched.file?
     mktemp do
       @downloader.stage
       if block_given?