Do not try to parse header on headerless formats

Fixes rb#392
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/formats.py

@@ -901,6 +901,8 @@ class PoFormat(FileFormat):
 
     def merge_header(self, otherstore):
         """Tries to merge headers"""
+        if not hasattr(self.store, 'updateheader'):
+            return
         values = otherstore.store.parseheader()
         skip_list = (
             'Plural-Forms',