Use the same API to add size to numeric input

Also comment why we are setting the :size attr to nil in numeric inputs, due to
how Rails handles this attribute. Need to check later whether to fix this on
Rails itself.

lib/simple_form/inputs/numeric_input.rb

@@ -4,7 +4,7 @@ module SimpleForm
       enable :placeholder
 
       def input
-        input_html_options[:size] ||= nil
+        add_size!
         if SimpleForm.html5
           input_html_options[:type] ||= "number"
           input_html_options[:step] ||= integer? ? 1 : "any"
@@ -19,6 +19,11 @@ module SimpleForm
 
       private
 
+      # Rails adds the size attr by default, if the :size key does not exist.
+      def add_size!
+        input_html_options[:size] ||= nil
+      end
+
       def infer_attributes_from_validations!
         return unless has_validators?