test: fix 3 blind spots reported by cargo-mutants

src/body/base64.rs

@@ -70,7 +70,7 @@ pub fn encode(b: &[u8], w: &mut dyn Write) -> fmt::Result {
 pub fn encoded_len(input_len: usize) -> usize {
     let mut base64_len = input_len / 3 * 4;
     if input_len % 3 != 0 {
-        base64_len += 4 - base64_len % 4;
+        base64_len += 4;
     }
     let mut crlf_len = base64_len / LINE_LEN * CRLF.len();
     if crlf_len >= CRLF.len() && base64_len % LINE_LEN == 0 {

src/body/chooser.rs

@@ -226,7 +226,13 @@ mod tests {
 
     #[test]
     fn not_too_long_multiline() {
-        let input = concat!("0123\n", "4567").as_bytes();
+        let input = concat!(
+            "0123\n",
+            "4567\n",
+            "00000000000000000000000000000000000000000\n",
+            "89"
+        )
+        .as_bytes();
 
         assert!(!line_too_long(input));
     }

src/headers/rfc2231.rs

@@ -372,4 +372,12 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    #[should_panic(expected = "`key` must only be composed of ascii alphanumeric chars")]
+    fn non_ascii_key() {
+        let mut s = String::new();
+        let mut w = EmailWriter::new(&mut s, 0, 0, true);
+        let _ = encode("📬", "", &mut w);
+    }
 }