signature_parameters_spec.
SigningString#to_str instead of #to_s
HeaderList uses #to_str not #to_s
SignatureParametersParser
Signer minor refactor.

Working towards extracting code necessary for signature verification.
Signer: remove unused error classes.
Signature extracted from Signer.
signature_parameters_spec has describe "#to_str" block.
Verifier and Verification.
Context#verifier