ProtoJSON does not encode bytes when strings contain non-ASCII-8BIT

[stuppy]

ProtoJSON support added #192. Temporalio::Api::Common::V1::Payload's data payload expects bytes, which require special conversion from JSON.

Example:

it’ll work!
  ^

raises:

Encoding::UndefinedConversionError:
  U+2019 from UTF-8 to ASCII-8BIT