into_inner on Decompressor discards data due to buffering

Steps to reproduce:

1. Decompress a Brotli stream from some input data.
2. Call `into_inner()`, then read() the result to check if there is any remaining unread input data (for instance, extra junk at the end of the compressed stream).

Expected result:

If there is unread input data, it is readable.

Actual result:

If there is unread input data, it may or may not be available after calling `into_inner()`, depending on whether it was in the buffer or not.

Proposed fix:

If a fix for #83 is implemented, Decompressor will have the property that reading `Ok(0)` from it means the entire input data was consumed. In that situation, `into_inner()` can safely be called. Decompressor's `into_inner()` could be documented to only be safe after reading `Ok(0)` from Decompressor.

This would be useful in the [ureq HTTP client](https://github.com/algesten/ureq/), where the inner reader might be a PoolReturnRead that returns a connection to a connection pool once a response body has been fully read. In that situation, we need to check for any unread bytes at the end of the response body, which could indicate a desynchronized HTTP connection that we need to drop.

Test code to reproduce:

```
#[test]
fn test_no_vanishing_bytes() {
// Output from this command:
// (brotli -c <<<"hello"; echo goodbye) | xxd -p | sed 's/../\\x&/g'
let compressed_with_extra = b"\x8f\x02\x80\x68\x65\x6c\x6c\x6f\x0a\x03\x67\x6f\x6f\x64\x62\x79\x65\x0a";
let mut cursor = Cursor::new(compressed_with_extra);
let mut reader = brotli_decompressor::Decompressor::new(cursor, 8000);
std::io::read_to_string(&mut reader).unwrap();
let unwrapped_cursor = reader.into_inner();
assert_eq!(std::io::read_to_string(unwrapped_cursor).unwrap(), "goodbye".to_string());
}
```