fix: clamp suffix ranges exceeding file size instead of returning 416

RFC 7233 Section 2.1 states that if the selected representation is
shorter than the specified suffix-length, the entire representation
is used.

`bytes=-999` on a 500-byte file now returns `bytes 0-499/500` (206)
instead of 416.

Author: mi-caren

src/headers/tests.rs

@@ -480,9 +480,9 @@ mod file_range {
     }
 
     #[test]
-    fn suffix_exceeds_size() {
-        let result = file_range(size(10), Some(HttpRange::Suffix(11)));
-        assert!(result.is_err());
+    fn suffix_exceeds_size_is_clamped() {
+        let result = file_range(size(10), Some(HttpRange::Suffix(11))).unwrap();
+        assert_eq!(result.range(), &(0..=9));
     }
 
     #[test]

src/lib.rs

@@ -58,7 +58,7 @@ pub fn file_range(
         HttpRange::Range(range) if range.start() < size => {
             range.start()..=range.end().min(size - 1)
         }
-        HttpRange::Suffix(suffix) if suffix > 0 && suffix <= size => size - suffix..=size - 1,
+        HttpRange::Suffix(suffix) if suffix > 0 => size.saturating_sub(suffix)..=size - 1,
         _ => {
             let content_range = HttpContentRange::Unsatisfiable(Unsatisfiable::new(size));
             return Err(UnsatisfiableRange(content_range));