Raise error when attempting to set a readonly field

lib/capybara/node/element.rb

@@ -97,6 +97,8 @@ module Capybara
       def set(value, options={})
         options ||= {}
 
+        raise Capybara::ReadOnlyElementError, "Attempt to set readonly element with value: #{value}" if readonly?
+
         driver_supports_options = (base.method(:set).arity != 1)
 
         unless options.empty? || driver_supports_options

lib/capybara/selenium/node.rb

@@ -225,9 +225,7 @@ private
   end
 
   def set_text(value, options)
-    if readonly?
-      warn "Attempt to set readonly element with value: #{value} \n *This will raise an exception in a future version of Capybara"
-    elsif value.to_s.empty? && options[:clear].nil?
+    if value.to_s.empty? && options[:clear].nil?
       native.clear
     else
       if options[:clear] == :backspace