Bug Regression - AWS IAM auth 'Content-Length' header causes validation error when sent as number instead of string or []strings

[aantono]

It appears that the 1.0 line has introduced the regression to the #3763

With 1.0.2, when doing AWS IAM auth using STS headers, if Content-Length value is a number instead of a string, i.e. 43, the bug resurfaces again. Given that the headers content comes from AWS SDK, it is not feasible to change that (short of manually re-parse every value and convert non-strings to strings, which is tedious).

[aantono]

Linking #2810

[aantono]

Relevant areas in the code:
https://github.com/hashicorp/vault/blob/v1.0.2/builtin/credential/aws/path_login.go#L1167
https://github.com/hashicorp/vault/blob/v1.0.2/logical/framework/field_data.go#L46
https://github.com/hashicorp/vault/blob/v1.0.2/logical/framework/field_data.go#L359