Fix type mismatch for reading Lists of Bytes

At the moment when writing Byte strings into tiff metadata sections (like
for XMP metadata) and try to read it with this crate, you will find your
characters as a Value::UnsignedBig, holding the char value with padding.

This seems wrong and we should instead return actual Value::Bytes.

src/decoder/ifd.rs

@@ -577,10 +577,10 @@ impl Entry {
             Type::BYTE => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
                 let mut buf = [0; 1];
                 reader.inner().read_exact(&mut buf)?;
-                Ok(UnsignedBig(u64::from(buf[0])))
+                Ok(Byte(buf[0]))
             }),
             Type::SBYTE => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(SignedBig(i64::from(reader.read_i8()?)))
+                Ok(SignedByte(reader.read_i8()?))
             }),
             Type::SHORT => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
                 Ok(Short(reader.read_u16()?))