MD5 calculation of SQS message attributes

This implements the same MD5 hashing pattern as implemented in the Ruby
and Java AWS SDKs

Doesn't yet handle list types but if you're reading this you might be
surprised how easy that is to add. Give it a shot and if you get stuck
reach out to me for help.

tests/test_sqs/test_sqs.py

@@ -43,10 +43,44 @@ def test_get_inexistent_queue():
 def test_message_send():
     sqs = boto3.resource('sqs', region_name='us-east-1')
     queue = sqs.create_queue(QueueName="blah")
-    msg = queue.send_message(MessageBody="derp")
-
+    msg = queue.send_message(
+        MessageBody="derp",
+        MessageAttributes={
+            'timestamp': {
+                'StringValue': '1493147359900',
+                'DataType': 'Number',
+            }
+        }
+    )
     msg.get('MD5OfMessageBody').should.equal(
         '58fd9edd83341c29f1aebba81c31e257')
+    msg.get('MD5OfMessageAttributes').should.equal(
+        '235c5c510d26fb653d073faed50ae77c')
+    msg.get('ResponseMetadata', {}).get('RequestId').should.equal(
+        '27daac76-34dd-47df-bd01-1f6e873584a0')
+    msg.get('MessageId').should_not.contain(' \n')
+
+    messages = queue.receive_messages()
+    messages.should.have.length_of(1)
+
+
+@mock_sqs
+def test_message_with_complex_attributes():
+    sqs = boto3.resource('sqs', region_name='us-east-1')
+    queue = sqs.create_queue(QueueName="blah")
+    msg = queue.send_message(
+        MessageBody="derp",
+        MessageAttributes={
+            'ccc': {'StringValue': 'testjunk', 'DataType': 'String'},
+            'aaa': {'BinaryValue': b'\x02\x03\x04', 'DataType': 'Binary'},
+            'zzz': {'DataType': 'Number', 'StringValue': '0230.01'},
+            'öther_encodings': {'DataType': 'String', 'StringValue': 'T\xFCst'}
+        }
+    )
+    msg.get('MD5OfMessageBody').should.equal(
+        '58fd9edd83341c29f1aebba81c31e257')
+    msg.get('MD5OfMessageAttributes').should.equal(
+        '8ae21a7957029ef04146b42aeaa18a22')
     msg.get('ResponseMetadata', {}).get('RequestId').should.equal(
         '27daac76-34dd-47df-bd01-1f6e873584a0')
     msg.get('MessageId').should_not.contain(' \n')