Don't nilify :opt arguments that are preceeding a :req argument

e.g.
def index(opt1 = 1, req1) end + params: {req1: 'aho'}
will pass in the default value 1 for "opt1" parameter
Extract params handling to a method

we firstly never imagined this logic to be such complicated, but it apparently needs thorough unit tests now
Extract StrongParameters handling to another method