Correct Binding#dup to properly propagate values it captures

[enebo]

Binding#dup will get all variables from the original binding but when new ones are added to the new dup the old binding will not see them. This is a more broken down spec for the behavior:

  it "retains original binding variables but the list is distinct" do
    bind1 = binding
    eval "a = 1", bind1

    bind2 = bind1.dup
    eval("a = 2", bind2)
    eval("a", bind1).should == 2
    eval("a", bind2).should == 2

    eval("b = 2", bind2)
    -> { eval("b", bind1) }.should raise_error(NameError)
    eval("b", bind2).should == 2
    
    bind1.local_variables.sort.should == [:a, :bind1, :bind2]
    bind2.local_variables.sort.should == [:a, :b, :bind1, :bind2]
  end

This fixes the last two errors involving specs in erb. I added this spec to make the behavior explicit and not appear to be some artifact of erb.