Fix #58: Checksum is not always 3 ASCII characters

Bug introduced in 9f5096a, where
I switched to using f-string formatting, but left off the left zero
pad for the checksum value.

This bug was present in v1.0.16.

simplefix/message.py

@@ -570,7 +570,7 @@ def encode(self, raw=False):
         checksum = 0
         for c in buf:
             checksum += c
-        buf += b"10=" + fix_val(f"{checksum % 256}") + SOH_STR
+        buf += b"10=" + fix_val(f"{checksum % 256:03}") + SOH_STR
 
         return buf
 

test/test_message.py

@@ -99,6 +99,19 @@ def test_raw_checksum(self):
         pkt.append_pair(10, 42)
         self.assertEqual(b"10=42\x01", pkt.encode(True))
 
+    def test_cooked_checksum(self):
+        """Test calculation of Checksum(10) in cooked mode"""
+        msg = FixMessage()
+        msg.append_pair(8, b'FIX.4.2')
+        msg.append_pair(35, b'D')
+        msg.append_pair(5001, b'AAAAA')
+        self.assertEqual(b"8=FIX.4.2\x01"
+                         b"9=16\x01"
+                         b"35=D\x01"
+                         b"5001=AAAAA\x01"
+                         b"10=048\x01",
+                         msg.encode())
+
     def test_raw_msg_type(self):
         """Test encoding of MessageType(35) in raw mode"""
         pkt = FixMessage()