srl_encoder.c: fix bug when sort_keys,stringify_unknown are combined

Bug introduced in commit 67e3d11a7239209e154c6b3b0ab4d192257f36af.
Looks like a copy/paste error.

Variable undef_unknown mistakenly set to 1 when option 'sort_keys'
is true. If 'stringify_unknown' is also true then we croak with
the message
  'undef_unknown' and 'stringify_unknown' options are mutually exclusive

The fix simply doesn't assign undef_unknown=1 when 'sort_keys' is true.
A test case t/021_sort_keys_option.t is also added.