Handle non-utf8 encodings in gRPC invoker

[scothis]

The gRPC invoker currently ignores the charset defined in the Content-Type header. We should attempt to use the declared charset when converting the node Buffer to a string. We'll likely need to limit the acceptable charsets since the Buffer type only supports a few encodings.

See https://nodejs.org/api/buffer.html#buffer_buffers_and_character_encodings

[jldec]

This is a followup for #30

[scothis]

no need with #65