Fix incorrect CRC on large files

The deflate writer did not always accept all data. The extra bytes did end up in the crc
calculation, so they where accounted for twice.

Resolves #8

src/write.rs

@@ -74,10 +74,16 @@ impl<W: Write+io::Seek> Write for ZipWriter<W>
     fn write(&mut self, buf: &[u8]) -> io::Result<usize>
     {
         if self.files.len() == 0 { return Err(io::Error::new(io::ErrorKind::Other, "No file has been started")) }
-        self.stats.update(buf);
         match self.inner.ref_mut()
         {
-            Some(ref mut w) => w.write(buf),
+            Some(ref mut w) => {
+                let write_result = w.write(buf);
+                if let Ok(count) = write_result {
+                    self.stats.update(&buf[0..count]);
+                }
+                write_result
+
+            }
             None => Err(io::Error::new(io::ErrorKind::BrokenPipe, "ZipWriter was already closed")),
         }
     }