print: fix str_sere() and str_utf8_sere(), as found by PVS-Studio

These were not actually printing in "SERE" mode due to a copy/paste
error.  PVS-Studio seems really good at finding those.  For #192.

* spot/tl/print.cc: Fix it.
* NEWS: Mention the bug.

NEWS

@@ -28,6 +28,9 @@ New in spot 2.1.2.dev (not yet released)
   * Compression could encode '6' and '22' in the same way. This issue
     only appears when using the compression from intvcomp.hh.
 
+  * str_sere() and str_utf8_sere() were not returning the same
+    string that print_sere() and print_utf8_sere() would print.
+
 New in spot 2.1.2 (2016-10-14)
 
   Command-line tools:

spot/tl/print.cc

@@ -893,7 +893,7 @@ namespace spot
   std::string
   str_sere(formula f, bool full_parent)
   {
-    return str_(f, full_parent, false, spot_kw);
+    return str_(f, full_parent, true, spot_kw);
   }
 
 
@@ -918,7 +918,7 @@ namespace spot
   std::string
   str_utf8_sere(formula f, bool full_parent)
   {
-    return str_(f, full_parent, false, utf8_kw);
+    return str_(f, full_parent, true, utf8_kw);
   }