Fix access to specializations when Selenium::Driver is subclassed

The specializations are stored in a class instance variable. However,
they are accessed from the specialize_driver method via `self.class`.

Previously if this method was called on an instance of a subclass of
Selenium::Driver, then it accessed the class instance variable
associated with the subclass (which is nil by default) and not the class
instance variable associated with the Selenium::Driver class as intended
resulting in an exception like this:

     NoMethodError:
       private method `select' called for nil:NilClass
       ./lib/capybara/selenium/driver.rb:417:in `specialize_driver'
       ./lib/capybara/selenium/driver.rb:49:in `browser'
       ./lib/capybara/selenium/driver.rb:66:in `visit'
       ./lib/capybara/session.rb:277:in `visit'
       ./spec/selenium_spec_chrome.rb:118:in `block (3 levels) in <top (required)>'

This commit adds a spec using a subclass of Selenium::Driver with Chrome
to demonstrate the problem and fixes it by changing the
specialize_driver method to access the specializations via the
explicitly specified class name.

An alternative fix would be to use a class variable for specializations
and do away with the class attribute reader altogether.

lib/capybara/selenium/driver.rb

@@ -414,7 +414,7 @@ private
 
   def specialize_driver
     browser_type = browser.browser
-    self.class.specializations.select { |k, _v| k === browser_type }.each_value do |specialization| # rubocop:disable Style/CaseEquality
+    Capybara::Selenium::Driver.specializations.select { |k, _v| k === browser_type }.each_value do |specialization| # rubocop:disable Style/CaseEquality
       extend specialization
     end
   end

spec/selenium_spec_chrome.rb

@@ -27,6 +27,11 @@ Capybara.register_driver :selenium_chrome_not_clear_storage do |app|
   Capybara::Selenium::Driver.new(app, chrome_options.merge(clear_local_storage: false, clear_session_storage: false))
 end
 
+Capybara.register_driver :selenium_driver_subclass_with_chrome do |app|
+  subclass = Class.new(Capybara::Selenium::Driver)
+  subclass.new(app, browser: :chrome, options: browser_options, timeout: 30)
+end
+
 module TestSessions
   Chrome = Capybara::Session.new(CHROME_DRIVER, TestApp)
 end
@@ -106,4 +111,12 @@ RSpec.describe 'Capybara::Session with chrome' do
       expect(Time.parse(extract_results(session)['datetime'])).to eq datetime
     end
   end
+
+  describe 'using subclass of selenium driver' do
+    it 'works' do
+      session = Capybara::Session.new(:selenium_driver_subclass_with_chrome, TestApp)
+      session.visit('/form')
+      expect(session).to have_current_path('/form')
+    end
+  end
 end