Wrap all compiler assignments in begin/end

This is much safer than having to explicitly call "wrap" in a potential
large amount of places.

spec/oga/ruby/generator_spec.rb

@@ -21,7 +21,11 @@ describe Oga::Ruby::Generator do
       val    = Oga::Ruby::Node.new(:lit, %w{10})
       assign = var.assign(val)
 
-      @generator.on_assign(assign).should == 'number = 10'
+      @generator.on_assign(assign).should == <<-EOF
+number = begin
+  10
+end
+      EOF
     end
   end
 

spec/oga/ruby/node_spec.rb

@@ -34,7 +34,11 @@ describe Oga::Ruby::Node do
       node  = left.assign(right)
 
       node.type.should == :assign
-      node.to_a.should == [left, right]
+
+      node.to_a[0].should == left
+
+      node.to_a[1].type.should    == :begin
+      node.to_a[1].to_a[0].should == right
     end
   end