encode and sign the ext option

[flitbit]

The signing spec/guide indicates that an optional extensions can be encoded in an http signature but the reference implementation did not enable such. This pull request modifies the signer to encode the ext option if specified by the caller. I also added a signer test and verify test for this scenario.

Regards.

[pfmooney]

Looking at the relevant section of the draft specification, I believe the extensions parameter is not meant to simply be included as a string value. Given that, I'm more inclined to change the documentation to state that extensions is reserved for future use and omit any implementation details for now.

[flitbit]

Well yes. I got in the lazy habit of referring to the markdown description rather than the draft because the markdown description is more human friendly. But I see what you mean and I will revise approach. It is a pity however that the draft has expired and there is no actual "Signature Authentication Scheme Registry" at IANA (that I can find).