Add tests for reading with an empty buffer, and fix bug in GzDecoder

[mdsteele]

I recently ran into a problem where I was wrapping a flate2::read::GzDecoder inside another reader, and the outer reader ended up calling read() on the GzDecoder with an empty buffer, and instead of just returning Ok(0), the GzDecoder returned an InvalidInput error with the message: "corrupt gzip stream does not have a matching checksum".

Here's a simple test case to reproduce (without the need for the outer writer):

extern crate flate2;

// Compress some data with gzip:
let original: &[u8] = b"Lorem ipsum dolor sit amet.";
let mut encoder = flate2::write::GzEncoder::new(Vec::new(), flate2::Compression::default());
encoder.write_all(original).unwrap();
let encoded: Vec<u8> = encoder.finish().unwrap();

// Now decompress it again:
let mut decoder = flate2::read::GzDecoder::new(encoded.as_slice());
assert_eq!(decoder.read(&mut []).unwrap(), 0);  // This call to read() fails
let mut decoded = Vec::new();
decoder.read_to_end(&mut decoded).unwrap();
assert_eq!(decoded.as_slice(), original);

The contract for Read::read() doesn't seem to require implementations to return Ok(0) when the buffer is empty (instead of blocking or returning an error), but it seems like the nice thing to do. Even if GzDecoder wanted to return an error in this case, "corrupt gzip stream does not have a matching checksum" is definitely misleading (and in fact, it took me some time to track down the cause).

Interestingly, the other Read impls in flate2 seem to handle read(&mut []) just fine (e.g. if you replace Gz with Zlib in the above test); it's just GzDecoder that breaks.

This PR adds a simple 3-line fix for GzDecoder, along with a bunch of unit tests.

[alexcrichton]

Awesome, thanks!