make the Cask DSL resilient to unexpected methods

will help with introducing and removing features, since the Cask
definitions move with every `brew update` and track master but the code
to handle them requires an explicit release and a `brew upgrade brew-cask`

relates to #179

lib/cask/dsl.rb

@@ -14,10 +14,6 @@ module Cask::DSL
   def sums; self.class.sums || []; end
 
   module ClassMethods
-    def content_length(content_length=nil)
-      # deprecated, but retained for backwards compatibility
-    end
-
     def homepage(homepage=nil)
       @homepage ||= homepage
     end
@@ -50,5 +46,9 @@ module Cask::DSL
     def no_checksum
       @sums = 0
     end
+
+    def method_missing(method, *args)
+      opoo "Unexpected method #{method} called on #{self}. Running `brew update; brew upgrade brew-cask` will likely fix it."
+    end
   end
 end

test/cask/dsl_test.rb

@@ -26,11 +26,28 @@ describe Cask::DSL do
   it "still lets you set content_length even though it is deprecated" do
     OldContentLengthCask = Class.new(Cask)
     begin
-      OldContentLengthCask.class_eval do
-        content_length '12345'
+      shutup do
+        OldContentLengthCask.class_eval do
+          content_length '12345'
+        end
       end
     rescue Exception => e
       flunk("expected content_length to work, but got exception #{e}")
     end
   end
+
+  it "prevents the entire world from crashing when a cask includes an unknown method" do
+    UnexpectedMethodCask = Class.new(Cask)
+    begin
+      lambda {
+        UnexpectedMethodCask.class_eval do
+          future_feature :not_yet_on_your_machine
+        end
+      }.must_output(
+        "Warning: Unexpected method future_feature called on UnexpectedMethodCask. Running `brew update; brew upgrade brew-cask` will likely fix it.\n"
+      )
+    rescue Exception => e
+      flunk("Wanted unexpected method to simply warn, but got exception #{e}")
+    end
+  end
 end