Add tests to show BufRead can be used after decoding

Copy the test from `gzip::bufread` to demonstrate that it also works
for `zlib` and `deflate`.

src/deflate/bufread.rs

@@ -243,3 +243,50 @@ impl<W: BufRead + Write> Write for DeflateDecoder<W> {
         self.get_mut().flush()
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::bufread::DeflateDecoder;
+    use crate::deflate::write;
+    use crate::Compression;
+    use std::io::{Read, Write};
+
+    // DeflateDecoder consumes one deflate archive and then returns 0 for subsequent reads, allowing any
+    // additional data to be consumed by the caller.
+    #[test]
+    fn decode_extra_data() {
+        let expected = "Hello World";
+
+        let compressed = {
+            let mut e = write::DeflateEncoder::new(Vec::new(), Compression::default());
+            e.write(expected.as_ref()).unwrap();
+            let mut b = e.finish().unwrap();
+            b.push(b'x');
+            b
+        };
+
+        let mut output = Vec::new();
+        let mut decoder = DeflateDecoder::new(compressed.as_slice());
+        let decoded_bytes = decoder.read_to_end(&mut output).unwrap();
+        assert_eq!(decoded_bytes, output.len());
+        let actual = std::str::from_utf8(&output).expect("String parsing error");
+        assert_eq!(
+            actual, expected,
+            "after decompression we obtain the original input"
+        );
+
+        output.clear();
+        assert_eq!(
+            decoder.read(&mut output).unwrap(),
+            0,
+            "subsequent read of decoder returns 0, but inner reader can return additional data"
+        );
+        let mut reader = decoder.into_inner();
+        assert_eq!(
+            reader.read_to_end(&mut output).unwrap(),
+            1,
+            "extra data is accessible in underlying buf-read"
+        );
+        assert_eq!(output, b"x");
+    }
+}

src/zlib/bufread.rs

@@ -251,3 +251,50 @@ impl<R: BufRead + Write> Write for ZlibDecoder<R> {
         self.get_mut().flush()
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::bufread::ZlibDecoder;
+    use crate::zlib::write;
+    use crate::Compression;
+    use std::io::{Read, Write};
+
+    // ZlibDecoder consumes one zlib archive and then returns 0 for subsequent reads, allowing any
+    // additional data to be consumed by the caller.
+    #[test]
+    fn decode_extra_data() {
+        let expected = "Hello World";
+
+        let compressed = {
+            let mut e = write::ZlibEncoder::new(Vec::new(), Compression::default());
+            e.write(expected.as_ref()).unwrap();
+            let mut b = e.finish().unwrap();
+            b.push(b'x');
+            b
+        };
+
+        let mut output = Vec::new();
+        let mut decoder = ZlibDecoder::new(compressed.as_slice());
+        let decoded_bytes = decoder.read_to_end(&mut output).unwrap();
+        assert_eq!(decoded_bytes, output.len());
+        let actual = std::str::from_utf8(&output).expect("String parsing error");
+        assert_eq!(
+            actual, expected,
+            "after decompression we obtain the original input"
+        );
+
+        output.clear();
+        assert_eq!(
+            decoder.read(&mut output).unwrap(),
+            0,
+            "subsequent read of decoder returns 0, but inner reader can return additional data"
+        );
+        let mut reader = decoder.into_inner();
+        assert_eq!(
+            reader.read_to_end(&mut output).unwrap(),
+            1,
+            "extra data is accessible in underlying buf-read"
+        );
+        assert_eq!(output, b"x");
+    }
+}