Add support for UNDEFINED type

According to TIFF spec this is an byte-array that may contain arbitrary
data.

src/decoder/ifd.rs

@@ -425,7 +425,10 @@ impl Entry {
                         let string = decoder.read_string(n)?;
                         Ok(Ascii(string))
                     }
-                }
+                },
+                (Type::UNDEFINED, n) => self.decode_offset(n, bo, limits, decoder, |decoder| {
+                    Ok(Unsigned(u32::from(decoder.read_byte()?)))
+                }),
                 _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::UnsupportedDataType,
                 )),

src/tags.rs

@@ -130,6 +130,7 @@ pub enum Type(u16) {
     LONG = 4,
     RATIONAL = 5,
     SBYTE = 6,
+    UNDEFINED = 7,
     SSHORT = 8,
     SLONG = 9,
     SRATIONAL = 10,