Avoid stack level too deep when decrypting objects

[cgunther]

As demonstrated in the following file, a stack level too deep error seems possible in the ObjectHash#decrypt method
https://www.dropbox.com/s/4lvx469c580iuiv/pdf-reader%20decrypt%20stack%20overflow.pdf?dl=0

The stacktrace I got trying to access the first page (pages.first) was:

SystemStackError: stack level too deep
  from pdf-reader/lib/pdf/reader/object_hash.rb:318:in `decrypt'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `block in decrypt'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `each'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `map'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `decrypt'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `block in decrypt'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `each'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `map'
  from pdf-reader/lib/pdf/reader/object_hash.rb:317:in `decrypt'
  from pdf-reader/lib/pdf/reader/object_hash.rb:84:in `[]'
  from pdf-reader/lib/pdf/reader/object_hash.rb:98:in `object'
  from pdf-reader/lib/pdf/reader/object_hash.rb:343:in `get_page_objects'
  from pdf-reader/lib/pdf/reader/object_hash.rb:348:in `block in get_page_objects'
  from pdf-reader/lib/pdf/reader/object_hash.rb:348:in `map'
  from pdf-reader/lib/pdf/reader/object_hash.rb:348:in `get_page_objects'
  from pdf-reader/lib/pdf/reader/object_hash.rb:243:in `page_references'
... 4 levels...
  from pdf-reader/lib/pdf/reader.rb:176:in `map'
  from pdf-reader/lib/pdf/reader.rb:176:in `pages'
  from (irb):1

This error was seen on v2.0.0 as well as master.

This PR probably shouldn't be merged directly. My intention was to demonstrate the error and take a basic pass at fixing it, but I'm not really sure what the #decrypt method does or what it's arguments represent. I mainly looked at #198 and tried to apply similar changes. I'm not sure how to come up with a minimal test case as you did in #198 to add to the test suite, or what exactly to test other than "doesn't error".

Thanks for pdf-reader!