Ruby warning fix: instance variable used before initialize

[ghost]

Fixes #62

[coveralls]

Coverage remained the same at 0.0% when pulling af896b0 on chaymaeBZ:ruby-warning-uninstialized-instance-variable into 5b1bb34 on savonrb:master.

[bvicenzo]

@chaymaeBZ @olleolleolle To simplify this code, we could use the Enumerable#group_by method... Wdyta?

@sections ||= document.root.element_children.group_by { |node| node.name }

[olleolleolle]

I think that sounds fine, if you make a PR, you can see if all tests still pass, would be nice.

[bvicenzo]

@olleolleolle Done: #90