This change marks the methods which construct user middleware, so they are
called with the 2.6 keyword argument behaviour in 2.7 (and avoid printing a
warning about it).

Some users add their own middleware & write their own constructors with named
parameters, and use keyword arguments when adding their middleware to the
builder:

```
class MyMiddleware < Faraday::Middleware
  def initialize(app, my_argument:)
    @app = app
    @my_argument = my_argument
  end

[...]

connection = Faraday.new() do |configuration|
  configuration.use MyMiddleware, my_argument: 'hello'
end
```

This works great in 2.6 & 2.7, but 2.7 prints a deprecation warning:
```
/Users/danielholz/.gem/ruby/2.7.0/gems/faraday-1.0.1/lib/faraday/dependency_loader.rb:21: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/danielholz/code/some_project/lib/some_project/faraday_middleware.rb:2: warning: The called method `initialize' is defined here
```
The warning is a bit frustrating, since the calling code is not in the user's
code, so they cannot add `**` to the call. The best option for them is to
change their middleware to accept a Hash as a positional parameter & pass a
Hash when adding their middleware (and checking for required parameters
themselves).

I tried an alternative to `ruby2_keywords` by adding an explicit `**kwargs`
argument to `Faraday::RackBuilder#use`, `Faraday::RackBuilder#use_symbol`,
`Faraday::RackBuilder::Handler#initialize`, and
`Faraday::DependencyLoader#new`, but it hit the explicit delegation of keyword
arguments issue
(https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/#why-deprecated)
in 2.6.

Co-authored-by: Olle Jonsson <olle.jonsson@gmail.com>

…b classes in spec

We don't have a specific base class for Request middleware like we have for response ones (Faraday::Response::Middleware), so cat_request should probably inherit from Faraday::Middleware instead.