AWS IAM header is incorrect

[jamesvl]

AWS authentication is sending a server header named X-Vault-awsiam-Server-Id when it should be X-Vault-AWS-IAM-Server-ID. (note the missing dash)

I can open a PR for this if you like, but it's just a one-character change.

I get when trying to use AWS IAM unless I fix the header myself:

{:error, ["error validating X-Vault-AWS-IAM-Server-ID header: missing header \"X-Vault-AWS-IAM-Server-ID\""]}