don't stream certain in-memory io.Readers

[joeshaw]

bytes.Buffer, bytes.Reader, and strings.Reader are all io.Reader
implementations on an in-memory buffer. Their sizes are known ahead of
time, and these types tend to be used for small payloads. We can send
these payloads with a Content-Length rather than using chunked
encoding by sending them with SendAsync rather than
SendAsyncStreaming.

[joeshaw]

Marked this as a draft because I'm not 100% convinced we definitely want to do this, but it's the implementation following my comment in #38 (comment).

[joeshaw]

One possible other adjustment would be to actually examine the size of the buffers (all three types implement Len() int) and still stream if over some size threshold. I'm not sure how much that matters though.

[joeshaw]

I decided not to limit it based on length because the Rust SDK doesn't do anything like that currently. (It implements the From trait for several string/byte slice/byte vector types and they also just copy.) However, if we wanted to add it later, we could add a limit for it, set it to a sensible default (64k?) and only copy if it's within the limit.

[joeshaw]

Moving back to draft, as this needs some work.