Formula#fetch: always return the cached path

This allows us to remove some type checks that were protecting against
potential nils.

Library/Homebrew/cmd/fetch.rb

@@ -27,8 +27,7 @@ module Homebrew extend self
     f.cached_download.rmtree if already_fetched?(f) && ARGV.force?
     download = f.fetch
 
-    # FIXME why are strategies returning different types?
-    return unless download.is_a? Pathname
+    return unless download.file?
 
     puts "Downloaded to: #{download}" unless already_fetched?(f)
     puts Checksum::TYPES.map { |t| "#{t.to_s.upcase}: #{download.send(t)}" }

Library/Homebrew/formula.rb

@@ -619,6 +619,7 @@ class Formula
     # Ensure the cache exists
     HOMEBREW_CACHE.mkpath
     downloader.fetch
+    cached_download
   end
 
   # For FormulaInstaller.
@@ -644,7 +645,7 @@ class Formula
 
   def stage
     fetched = fetch
-    verify_download_integrity(fetched) if fetched.kind_of? Pathname
+    verify_download_integrity(fetched) if fetched.file?
     mktemp do
       downloader.stage
       # Set path after the downloader changes the working folder.