Allow use of instance variables for advanced memoization in Rails/HelperInstanceVariable

[wscourge]

Is your feature request related to a problem? Please describe.

When I use advanced memoization techniques in a helper method, it gets flagged by Rails/HelperInstanceVariable.

Describe the solution you'd like

I'd like Rails/HelperInstanceVariable to accept assignment to instance variables for memoization purposes, i.e., it should allow methods like the following:

def foo
  return @foo if defined?(@foo)

  @foo ||= expensive_calculation_of_foo_that_can_actually_return_nil_value(bar, baz)
end

Describe alternatives you've considered

I considered moving the method elsewhere, but it doesn't make sense for its location to be dependent on using memoization or not.

Additional context

Shamelessly extended the following: #311