Add HTTP::Request#form_params

[threez]

The current API has helpers to get the query params for an HTTP request. I was searching io how to handle
a regular http post. I did not find an direct answer, again searching the docs. After a little while I noticed that there are outdated examples on stack overflow. To safe everybody else the hassle of searching how to get form params, I added
convenience helpers in the spirit of query_params.

This will enable uses like this:

require "http/server"

class SomeHandler
  include HTTP::Handler

  def call(context)
    if params = context.request.form_params?
      # do something with
      puts params["some"]?
    end
  end
end

[straight-shoota]

@threez Would you mind adressing the review comment?
If you don't want to continue with this PR, please let us know and someone else will take over.

[threez]

@straight-shoota, sorry for late reply. I added the changes as requested.