Use correct methods when raising error during signing/verification with EdDSA

[hieuk09]

Description

This Pull Request changes/fixes this error when the key for EdDSA is invalid

NoMethodError:
  undefined method `raise_encode_error!' for an instance of JWT::JWA::Eddsa

This method is renamed to raise_sign_error!. This PR updates raise_decode_error! to raise_verify_error!, too.

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

[anakinj]

Great job. Thank you.