Force UTF-8 encoding of history entry strings

Fixes 139

lib/rbreadline.rb

@@ -6131,6 +6131,7 @@ module RbReadline
   end
   def alloc_history_entry(string, ts)
+    string = string.encode('UTF-8', invalid: :replace, undef: :replace, replace: '')
     temp = Struct.new(:line,:data,:timestamp).new
     temp.line = string ? string.delete(0.chr) : string
     temp.data = nil