Handle HTTP chunking across multi-byte boundaries

[Freaky]

When a response contains multi-byte UTF-8 characters that cross chunk boundaries, appending the two halves to the intermediate string results in two Unicode Replacement Characters being inserted instead of the two halves being joined.

Use StringDecoder to buffer the partial characters across chunks.

Resolves #22

[Freaky]

CI failures look to be a result of https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

Instructing it to use OpenSSL's doesn't appear to help, hmm.

[zyc9012]

Thank you @Freaky. I fixed the CI and squashed my commit with yours.