Set header Content-Length as a string

[DaniGuardiola]

The number format caused a problem on the node-vault module: hashicorp/vault#3763 (comment)

This should solve it, unless there's any reason to keep it as a number.

[DaniGuardiola]

I tested the code and it works just fine

[mhart]

I think this is best done in the vault code (considering that they're the ones enforcing the schema) – that way it doesn't affect any existing users of aws4.

ie, do it just before here: https://github.com/DaniGuardiola/node-vault/blob/7525dc5a335d70be21e3cfd77becfc4bf9e9c94a/src/aws-auth.js#L56

[DaniGuardiola]

@mhart

But doesn't the signature need the same header format? Just wondering.