[ruby/reline] If history-size isn't numeric, it should be 500

https://tiswww.case.edu/php/chet/readline/readline.html#IDX25

> If an attempt is made to set history-size to a non-numeric value, the maximum
> number of history entries will be set to 500.

https://github.com/ruby/reline/commit/acf0437280

lib/reline/config.rb

@@ -210,7 +210,11 @@ class Reline::Config
   def bind_variable(name, value)
     case name
     when 'history-size'
-      @history_size = value.to_i
+      begin
+        @history_size = Integer(value)
+      rescue ArgumentError
+        @history_size = 500
+      end
     when 'bell-style'
       @bell_style =
         case value