Add QueryParser#missing_value for handling missing values + tests.

[ioquatix]

This is required for customizing how Rack::QueryParser handles missing values and make it easy for sub-classes to change this behaviour. Based on #2038 (comment).

For the purpose of Rails:

module ActionDispatch
  class Request
    # ...
    if Rack.release >= "3"
      class QueryParser < ::Rack::QueryParser
        def missing_value
          nil
        end
      end

      def query_parser
        QueryParser
      end
    end

A different design option would be to make this an argument, e.g. QueryParser.new(missing_value: nil).

cc @matthewd @tenderlove

[jeremyevans]

I'm fine with this in principle. One inline comment.

[jeremyevans]

This seems unrelated to the change being made. I'm not necessary opposed to this, but it shouldn't be part of this PR. It can be a separate PR with a reason given for the change. Ditto for the uri require at the top.

[ioquatix]

I couldn't make the separate tests pass without it, otherwise it introduces a circular dependency. I don't want to make a separate PR as we plan to backport this AFAIK.

[jeremyevans]

Fix for this is simple, submitted in #2055. However, considering this was backported without any discussion, I'm not sure what we want to do here.