Fix EOF not being handled correctly

[sommd]

In IOReader and EIOReader, pop() does not properly handle EOF. The read() methods return Ok(0) when reaching the end of the file:

• https://doc.rust-lang.org/std/io/trait.Read.html#tymethod.read
• https://docs.rs/embedded-io/latest/embedded_io/trait.Read.html#tymethod.read

Switched to using read_exact(), which returns an Err if EOF is reached, which is what the code seems to expect.

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[max-heller]

Makes sense, could you add a test covering this?

[sommd]

Wasn't sure where you'd want to put integration tests (didn't feel like they'd fit in loopback), so I just added some unit tests. If you want integration tests, just let me know where I should put them 🙂

[max-heller]

Looks good, thanks for the contribution!