Raise an exception if a downloader doesn't support :head.

lib/cocoapods/installer.rb

@@ -74,6 +74,7 @@ module Pod
         if downloader.respond_to?(:download_head)
           downloader.download_head
         else
+          raise Informative, "The downloader of class `#{downloader.class.name}' does not support the `:head' option."
         end
       else
         downloader.download

spec/functional/downloader_spec.rb

@@ -261,7 +261,7 @@ module Pod
       end
     end
 
-    describe "for Http" do
+    describe "for HTTP" do
       extend SpecHelper::TemporaryDirectory
 
       it "download file and unzip it" do