Function validate_header 

pub fn validate_header<E>(header: &Header<E>) -> Result<(), OperationError>
where
    E: Extensions,

Validate an operation header.

This method validates that the following conditions are true:

• Signature can be verified against the author public key and unsigned header bytes
• Header version is supported (currently only version 1 is supported)
• If payload_hash is set the payload_size is > 0 otherwise it is zero
• If backlink is set then seq_num is > 0 otherwise it is zero