Fix crash on invalid rails6-format cookie

[guisehn]

When providing an invalid cookie in Rails 6 format (with 3 parts), MessageEncryptor.authenticate_and_decrypt/2 could cause exceptions:

1. if any of the three parts was invalid base64, it would raise an ArgumentError like:

Elixir.ArgumentError non-alphabet character found: " " (byte 32) 
    lib/base.ex:137 Base.bad_character!/1
    lib/base.ex:649 Base."-decode64base!/2-lbc$^0/2-0-"/2
    lib/base.ex:640 Base.decode64base!/2
    lib/enum.ex:1658 Enum."-map/2-lists^map/1-0-"/2
    lib/plug_rails_cookie_session_store/message_encryptor.ex:74 PlugRailsCookieSessionStore.MessageEncryptor.authenticate_and_decrypt/3
    lib/plug_rails_cookie_session_store.ex:99 PlugRailsCookieSessionStore.get/3
    lib/plug/session.ex:78 anonymous fn/5 in Plug.Session.fetch_session/1

or:

Elixir.ArgumentError non-alphabet character found: " " (byte 32) 
    lib/base.ex:137 Base.bad_character!/1
    lib/base.ex:649 Base."-decode64base!/2-lbc$^0/2-0-"/2
    lib/base.ex:640 Base.decode64base!/2
    lib/enum.ex:1658 Enum."-map/2-lists^map/1-0-"/2
    lib/plug_rails_cookie_session_store/message_encryptor.ex:74 PlugRailsCookieSessionStore.MessageEncryptor.authenticate_and_decrypt/3
    lib/plug_rails_cookie_session_store.ex:99 PlugRailsCookieSessionStore.get/3
    lib/plug/session.ex:78 anonymous fn/5 in Plug.Session.fetch_session/1

2. if it was valid base64 but an invalid payload, it would return {:ok, :error} instead of {:error}, which would crash on:

Request: GET /
** (exit) an exception was raised:
    ** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not an iodata term

        :erlang.iolist_to_binary(:error)
        (jason 1.3.0) lib/jason.ex:69: Jason.decode/2
        (plug_rails_cookie_session_store 2.0.0) lib/plug_rails_cookie_session_store.ex:171: PlugRailsCookieSessionStore.decode/2
        (plug 1.13.6) lib/plug/session.ex:78: anonymous fn/5 in Plug.Session.fetch_session/1
        (plug 1.13.6) lib/plug/debugger.ex:287: Plug.Debugger.maybe_fetch_session/1
        (plug 1.13.6) lib/plug/debugger.ex:187: Plug.Debugger.render/6
        (plug 1.13.6) lib/plug/debugger.ex:162: Plug.Debugger.__catch__/5

We fixed to return just :error in both cases, which gracefully falls back to an empty session.

[guisehn]

Those tests were failing, padding-removal code was removed on 541b0c5 but tests were not updated