Switches where we check for invalid input, to move it inside the download function

[orta]

While CocoaPods would have seen this issue, moves the check into the core path of using the cocoapods-download api. We're supposed to assume clients would not always run the pre-process API:

      # preprocess download options
      #
      # Usage of this method is optional. concrete strategies should not
      # assume options are preprocessed for correct execution.
      #

[dnkoutso]

LGTM @orta want to do it for 1-6-stable so I can ship 1.6.3?

[orta]

Hrm, tests passed locally, let's see what's up

[dnkoutso]

I think it was just rubocop