Add middleware to handle unexpected non-json response for edge case

[nikushi]

Hi, recently I got NoMethodError exception during Marketo side system trouble. This exception happened on GET /identity/oauth/token due to following backtrace.

got #<NoMethodError: undefined method `fetch' for String> with backtrace:
             # ./lib/mrkt/concerns/authentication.rb:34:in `block in authenticate'
             # ./lib/mrkt/concerns/authentication.rb:31:in `tap'
             # ./lib/mrkt/concerns/authentication.rb:31:in `authenticate'

This is a bug of mrkt gem of edge case handling. There is a case that Marketo's server could respond with non json content-type e.g. text/plain. Although I could not confirm what it's content type was accurately, I believe that the exception was caused by the content type, from the stracktrace we got. You can see the detail in the first commit to reproduce the bug.

[nikushi]

Thanks for reviewing. Do you mean that the middleware I added should not pass 2XX response? Instead it should raise the same error too? I was worried about that point actually. If we choose more strict approach, it should be raised if the middleware see a response type other than json, even if it's status is 2XX or something successful one.