Fix for when DSL object is also the block's context

* fix #29

* added RSpec example for issue #29

* fixed a typo on the descripton comment

lib/docile/execution.rb

@@ -30,6 +30,7 @@ module Docile
         end
 
         block_context.instance_variables.each do |ivar|
+          next unless proxy_context.instance_variables.include?(ivar)
           value_from_dsl_proxy = proxy_context.instance_variable_get(ivar)
           block_context.instance_variable_set(ivar, value_from_dsl_proxy)
         end

spec/docile_spec.rb

@@ -349,6 +349,38 @@ describe Docile do
 
     end
 
+    context "when DSL context object is the same as the block's context object" do
+      class DSLContextSameAsBlockContext
+        def foo(v = nil)
+          @foo = v if v
+          @foo
+        end
+
+        def bar(v = nil)
+          @bar = v if v
+          @bar
+        end
+
+        def dsl_eval(block)
+          Docile.dsl_eval(self, &block)
+        end
+
+        def dsl_eval_string(string)
+          block = binding.eval("proc { #{string} }")
+          dsl_eval(block)
+        end
+      end
+
+      let(:dsl) { DSLContextSameAsBlockContext.new }
+
+      it "calls DSL methods and sets instance variables on the DSL conetxt object" do
+        dsl.dsl_eval_string('foo 0; bar 1')
+        expect(dsl.foo).to eq(0)
+        expect(dsl.bar).to eq(1)
+      end
+
+    end
+
   end
 
   describe ".dsl_eval_with_block_return" do