Fix continuation token invalid error

[mattprivman]

Listing S3 buckets with responses that require continuation tokens currently throws the exception below. This is typically only seen with S3 buckets with 1000's of objects to list.

Unhandled exception: InvalidArgument: The continuation token provided is incorrect (Awscr::S3::Http::ServerError)
  from lib/awscr-s3/src/awscr-s3/http.cr:109:9 in 'handle_response!'
  from lib/awscr-s3/src/awscr-s3/http.cr:86:7 in 'get'
  from lib/awscr-s3/src/awscr-s3/paginators/list_object_v2.cr:28:7 in 'next_response'
  from lib/awscr-s3/src/awscr-s3/paginators/list_object_v2.cr:23:22 in 'next'
  from /usr/local/Cellar/crystal/0.31.1/src/iterator.cr:442:7 in '__crystal_main'
  from /usr/local/Cellar/crystal/0.31.1/src/crystal/main.cr:97:5 in 'main_user_code'
  from /usr/local/Cellar/crystal/0.31.1/src/crystal/main.cr:86:7 in 'main'
  from /usr/local/Cellar/crystal/0.31.1/src/crystal/main.cr:106:3 in 'main'

Thanks to the investigation in this AWS forum post, this seems to be because of special characters in the obfuscated continuation token.

Passing token: 1TZkJICXgR0jZrdAHEJMonlSkBedCNzVrXzaGnw8ZBXsL5r5vhMtL7rJeJCTRvl1WYAoW27oMqx/JMbzac2uMqg==
Failing token: 1oDMtg+plckMHvLSZvcmKg6Gn5QYOEbCY9z/2gPv9gOXdHJRMSuQ/tibeJlExsXSBJwhmPaiDpMOQOF+c/49MmA==

Sometimes 1-3 requests pass through until it hits one with a plus in it and fails.

Running URI.encode with space_to_plus: true fixed this error.

Looking for feedback on this approach and whether it is safe to run space_to_plus: true on all the query params or if we should scope it to continuation-token.

[taylorfinnell]

I'm not sure if it's ok to use everywhere. Similar uncertainty in #34. I wonder if the ruby implementation would shed some light