Make sure there is only one instance of prodid, rev and uid

Multiple prodid causes vobject to crash at serialization, but there should
also be at most one instance of rev and uid, according to
https://tools.ietf.org/html/rfc6350, so check those properties as well

vcardlib.py

@@ -59,6 +59,8 @@
 OPTION_UPDATE_GROUP_KEY = True
 OPTION_FRENCH_TWEAKS = False
 
+SINGLE_INSTANCE_PROPERTIES = {'prodid', 'rev', 'uid'}
+
 def add_attributes(attributes, a):
     """
     Return void
@@ -382,6 +384,9 @@ def build_vcard(attributes):
 
     logging.debug("Building vcard ...")
     vcard = vCard()
+
+    defined_single_instance_properties = set()
+
     for a_name, a in attributes.items():
         #logging.debug("\tprocessing '%s' -> '%s'", a_name, a)
         if a:
@@ -430,6 +435,11 @@ def build_vcard(attributes):
                                 vcard.add(a_name).value = a_item
                                 logging.debug("\t%s: %s", a_name, a_item)
                             elif isinstance(a_item, ContentLine):
+                                if a_name in SINGLE_INSTANCE_PROPERTIES:
+                                    if a_name in defined_single_instance_properties:
+                                        continue
+                                    else:
+                                        defined_single_instance_properties.add(a_name)
                                 item_added = vcard.add(a_name)
                                 logging.debug("\t%s: %s", a_name, a_item.value)
                                 item_added.value = a_item.value