fix correct AMS header selection

When we are verifying the ARC seal we need to fetch the raw AMS header
from the header list. But it's not enough to return the first one we
find, since we may be interested in a different arc seal, we need
to search for the correct ARC index.

dkim/__init__.py

@@ -1293,7 +1293,9 @@ class ARC(DomainSigner):
     # we can't use the AMS provided above, as it's already been canonicalized relaxed
     # for use in validating the AS.  However the AMS is included in the AMS itself,
     # and this can use simple canonicalization
-    raw_ams_header = [(x, y) for (x, y) in self.headers if x.lower() == b'arc-message-signature'][0]
+    raw_ams_header = [
+       (x, y) for (x, y) in self.headers if x.lower() == b'arc-message-signature' and b" i="+sig[b'i']+b";" in y.lower()
+    ][0]
 
     # Only relaxed canonicalization used by ARC
     if b'c' not in sig: