Cop idea: redudant self assignment

[marcandre]

Since methods like Array#concat returns self:

# bad
args = args.concat(ary)
self.foo = foo.concat(ary)

# good
args.concat(foo) # or
args += foo
foo.concat(ary) # or
self.foo += ary

Noticed in our code

[marcandre]

Note: I didn't check if we already have a similar cop