Using credential_process results in a 404 error

When using an external credential process get-remote-codecommit always returns a 404.

I'm using aws-vault to test this, but your credential_process could be a cat of a json file.

I'm using aws-vault to handle my credentials, this is my configuration:

```
# .aws/config

[profile codecommit]
region=eu-west-1
credential_process = aws-vault exec sandbox-profile --json

[profile tmp]
region=eu-west-1
output=json
```

I verified that aws-vault was working and the credentials where correct by using `sts get-caller-identity` and testing the aws-vault credential output in a temporary profile:

```
aws --profile codecommit sts get-caller-identity
{
"UserId": "AROAIR7P4ZLYKGYCEVD6M:1583408935234570000",
"Account": "XXXXXXXXXXX",
"Arn": "arn:aws:sts::XXXXXXXXXXX:assumed-role/ROLE_NAME/1583408935234570000"
}

aws-vault exec sandbox-profile --json
{"Version":1,"AccessKeyId":"ASIAUBJLRPEKAZ2K7YC4","SecretAccessKey":"XXX","SessionToken":"YYY","Expiration":"2020-03-05T12:10:05Z"}

cat >> ~/.aws/credentails <