Permit empty "headers" signature parameter

[liamdennehy]

The specification permits an empty "headers" parameter for a signature, requiring signatures to be generated and validated against the default value date. Correct behaviour is required to implement the specification's reference tests (along with RSA). This PR:

• Modifies the validation to generate an implicit value of date for the signature parameter "header" when validating, and similarly for signing
• Accepts a missing "headers" parameter as a valid signature header line
• Does not emit the "headers" value onto the signature header line when signing a message if it has not been explicitly specified

[liamdennehy]

Any further issues or can we merge?

Note this is required to pass the reference tests, so hopefully this can go through.

[liamdennehy]

Any updates?

[liamdennehy]

I have integrated this feature in my own project along with a number of others, and published this in packagist. Documentation for the entire library is published at Read the Docs: http-signatures-php - incomplete but being expanded regularly.

[liamdennehy]

Closing PR due to inactivity.