Option: auto judge content type by `Data` field type

[wenchy]

See https://requests.readthedocs.io/en/latest/user/quickstart/#more-complicated-post-requests

We already supported form-data by providing Form option, much like the data in python requests. So
currently no "Content-Type" is auto injected. Just treat the Data as string body: d := fmt.Sprintf("%v", opts.Data).

• string or fmt.Stringer
• []byte

References

https://pkg.go.dev/fmt#Stringer