can't pass array argument to `secret`

Template: `{{with secret "/auth/token/create" "policies=policy1,policy2"}}{{.Auth.ClientToken}}{{ end }}`

### Expected behavior

What should have happened?

I should get a new token with policies "policy1" and "policy2".

### Actual behavior

I get an error, `* child policies must be subset of parent`. This is because my `policies` argument is being treated as a single policy with a comma in its name, rather than two separate policies.

### Discussion

The parsing code for the `secret` function treats argument values as strings. There's no way to pass an array as is expected by the `policies` argument to `auth/token/create`.

Start by tracing the parsing code for the `secret` function and how it passes arguments to `auth/token/create`. Verify how `policies=policy1,policy2` is represented before reaching the API, then check the relevant tests or add coverage for multiple policies. Done means the template produces a token with both policies rather than treating them as one name.

Written by the indexing model from the issue text.