adding a single comma at the end of the options list confuse the module

it start to duplicate line, because this create
a empty option in the list, and so the module add a new line along the previous
one. See

>>> ' a,b, '.strip().split(',')
['a', 'b', '']

library/system/authorized_key

@@ -185,6 +185,9 @@ def parseoptions(options):
     if options:
         options_list = options.strip().split(",")
         for option in options_list:
+            # happen when there is comma at the end
+            if option == '':
+                continue
             if option.find("=") != -1:
                 (arg,val) = option.split("=", 1)
             else: