Apply bip327: minor fixes

see: bitcoin/bips#1647

spec/fixtures/sig_agg_vectors.json

@@ -143,7 +143,8 @@
             ],
             "error": {
                 "type": "invalid_contribution",
-                "signer": 1
+                "signer": 1,
+                "contrib": "psig"
             },
             "comment": "Partial signature is invalid because it exceeds group size"
         }

spec/schnorr/musig2_spec.rb

@@ -96,6 +96,9 @@
         # The aggregate of the first three elements of pnonce is at index 0
         agg_nonce = described_class.aggregate_nonce(pub_nonces[0..2])
         expect(agg_nonce).to eq(agg_nonces[0].downcase)
+        # The aggregate of the first and fourth elements of pnonce is at index 1,
+        # which is the infinity point encoded as a zeroed 33-byte array
+        expect(agg_nonces[1].downcase).to eq(described_class.aggregate_nonce([pub_nonces[0], pub_nonces[3]]))
 
         vector['valid_test_cases'].each do |test|
           target_pubkeys = test['key_indices'].map {|i| pubkeys[i] }