use global context by default when invoking a fn.

lib/v8/function.rb

@@ -1,5 +1,6 @@
 class V8::Function < V8::Object
   include V8::Error::Try
+
   def initialize(native = nil)
     super do
       native || V8::C::FunctionTemplate::New().GetFunction()
@@ -8,6 +9,7 @@ class V8::Function < V8::Object
 
   def methodcall(this, *args)
     @context.enter do
+      this ||= @context.native.Global()
       @context.to_ruby try {native.Call(@context.to_v8(this), args.map {|a| @context.to_v8 a})}
     end
   end

spec/v8/function_spec.rb

@@ -0,0 +1,9 @@
+require 'spec_helper'
+
+describe V8::Function do
+  it "uses the global context if it is invoked with nil as the context" do
+    @cxt = V8::Context.new
+    @cxt['foo'] = 'bar'
+    @cxt.eval('(function() {return this.foo})').methodcall(nil).should eql 'bar'
+  end
+end