Add support for ASCII tags

tests/decode_images.rs

@@ -1,7 +1,7 @@
 extern crate tiff;
 
 use tiff::ColorType;
-use tiff::decoder::{Decoder, DecodingResult};
+use tiff::decoder::{Decoder, DecodingResult, ifd::Tag, ifd::Value};
 
 use std::fs::File;
 
@@ -45,6 +45,24 @@ fn test_rgb_u16()
     assert!(img_res.is_ok());
 }
 
+#[test]
+fn test_string_tags()
+{
+    // these files have null-terminated strings for their Software tag. One has extra bytes after
+    // the null byte, so we check both to ensure that we're truncating properly
+    let filenames = vec!["minisblack-1c-16b.tiff", "rgb-3c-16b.tiff"];
+    for filename in filenames.iter() {
+        let path = format!("./tests/images/{}", filename);
+        let img_file = File::open(path).expect("can't open file");
+        let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+        let software= decoder.get_tag(Tag::Software).unwrap();
+        match software {
+            Value::Ascii(s) => assert_eq!(s, String::from("GraphicsMagick 1.2 unreleased Q16 http://www.GraphicsMagick.org/")),
+            _ => assert!(false)
+        };
+    }
+}
+
 // TODO: GrayA support
 //#[test]
 //fn test_gray_alpha_u8()