[#Fix 334] Make body consistent for RescueNode, EnsureNode and KeywordBeginNode

[dvandersluis]

Renames EnsureNode#body to #branch, and adds a new #body method in line with RescueNode and KeywordBeginNode to return the body of code being executed before exception handling. For all three node types, body will now return a single node (which may be a begin/kwbegin node containing multiple nodes).

begin
  foo       <--- body
  bar       <--- body
rescue 
  baz
ensure
  quux
end

This is a breaking change, as existing code that calls EnsureNode#body will need to be updated.

Fixes #334.

[dvandersluis]

I'm not sure when the plan for the next major version is, but getting this kicked off now so it's ready for whenever that'll be.

[dvandersluis]

I used branch for the method to get the ensure branch body, similiar to how we have branches for RescueNode.

[dvandersluis]

I've gone through every rubocop-* repo in the @rubocop org. As far as I can see, only rubocop/rubocop and rubocop/rubocop-minitest are affected by this change. For tracking purposes, here are the PRs to update EnsureNode.body in those repos:

• Replace EnsureNode.body with EnsureNode.branch rubocop#13456
• Replace EnsureNode.body with EnsureNode.branch rubocop-minitest#324