Metadata API: Be more explicit when appending sigs

Clearing the OrderedDict makes it easier to see what happens and
avoids having to call OrderedDict() again.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

tuf/api/metadata.py

@@ -249,10 +249,10 @@ def sign(
 
         signature = signer.sign(signed_serializer.serialize(self.signed))
 
-        if append:
-            self.signatures[signature.keyid] = signature
-        else:
-            self.signatures = OrderedDict({signature.keyid: signature})
+        if not append:
+            self.signatures.clear()
+
+        self.signatures[signature.keyid] = signature
 
         return signature