Remove HTTPHeaders from ResponseMetadata in some tests

botocore 1.4.29 started adding this. I'd match against it, but it
contains a date object and it's just not worth rewriting the assertions
for a key-by-key approach.

tests/test_ec2/test_nat_gateway.py

@@ -56,6 +56,7 @@ def test_delete_nat_gateway():
     nat_gateway_id = nat_gateway['NatGateway']['NatGatewayId']
     response = conn.delete_nat_gateway(NatGatewayId=nat_gateway_id)
 
+    response['ResponseMetadata'].pop('HTTPHeaders') # this is hard to match against, so remove it
     response.should.equal({
         'NatGatewayId': nat_gateway_id,
         'ResponseMetadata': {