Bugfix: do not confuse Kernel#Array

On 1.9.2 (and maybe earlier) Kernel#Array will choke on a decorator,
because it just invokes `to_ary` and `to_a` without using `respond_to?`
first, like it does in 1.9.3-p125.

As a work-around, make sure to not create a delegation method in
`method_missing` when the model will not respond to it anyway. The
decorator object will then behave like `Kernel#Array` expects it to.

See #115.

lib/draper/base.rb

@@ -204,11 +204,13 @@ module Draper
     def method_missing(method, *args, &block)
       super unless allow?(method)
 
-      self.class.send :define_method, method do |*args, &block|
-        model.send method, *args, &block
-      end
+      if model.respond_to?(method)
+        self.class.send :define_method, method do |*args, &block|
+          model.send method, *args, &block
+        end
 
-      send method, *args, &block
+        send method, *args, &block
+      end
 
     rescue NoMethodError => no_method_error
       super if no_method_error.name == method

spec/draper/base_spec.rb

@@ -622,6 +622,10 @@ describe Draper::Base do
     end
 
     context "when the delegated method calls a non-existant method" do
+      it 'should not try to delegate to non-existant methods to not confuse Kernel#Array' do
+        Array(subject).should be_kind_of(Array)
+      end
+
       it "raises the correct NoMethodError" do
         begin
           subject.some_action