Ensure StreamSender enforces Exchange Rates

[sappenin]

See comments here: interledger/interledger-rs#273

Specifically, this TODO in the Rust code (which will have a corresponding entry int he Java code once committed to master).

Reference this portion of IL-RFC-29 for Exchange Rates.

[sappenin]

This fix required two changes. One to the StreamReceiver (fixed in #285) and a separate fix in the StreamSender.

The work for the StreamSender is more formally defined in #291, so we should consider closing this task in-favor of that one.

[sappenin]

See #291

[sappenin]

@theotherian there is a new comment in the associated RUST issue from Kincaid. Let’s look at our implementation to confirm we’re accounting for this use-case.