chore(clippy): fix some pedantic lints

src/body/base64.rs

@@ -46,7 +46,7 @@ pub fn encode(b: &[u8], w: &mut dyn Write) -> fmt::Result {
     while let Some(chunk) = chunks.next() {
         let len = ::base64::engine::general_purpose::STANDARD
             .encode_slice(chunk, &mut buf)
-            .unwrap();
+            .expect("base64 output `buf` is not big enough");
 
         w.write_str(str::from_utf8(&buf[..len]).expect("base64 produced an invalid encode"))?;
         if chunks.peek().is_some() {

src/headers/rfc2047.rs

@@ -54,7 +54,7 @@ pub fn encode(mut s: &str, w: &mut EmailWriter<'_>) -> fmt::Result {
 
             // No space remaining, but going to a new line will require us
             // to introduce a new space, which will mess up things even more.
-            word = &s[..s.chars().next().unwrap().len_utf8()];
+            word = &s[..s.chars().next().expect("`s` is empty").len_utf8()];
         }
 
         // Write the prefix

src/headers/rfc2231.rs

@@ -105,15 +105,15 @@ pub fn encode(key: &str, mut value: &str, w: &mut EmailWriter<'_>) -> fmt::Resul
 
                 w.write_char('"')?;
 
-                if !value.is_empty() {
-                    // End of line
-                    w.write_char(';')?;
-                    w.new_line()?;
-                } else {
+                if value.is_empty() {
                     // End of value
                     break;
                 }
 
+                // End of line
+                w.write_char(';')?;
+                w.new_line()?;
+
                 i += 1;
             }
         }
@@ -144,15 +144,15 @@ pub fn encode(key: &str, mut value: &str, w: &mut EmailWriter<'_>) -> fmt::Resul
                 }
             }
 
-            if !value.is_empty() {
-                // End of line
-                w.write_char(';')?;
-                w.new_line()?;
-            } else {
+            if value.is_empty() {
                 // End of value
                 break;
             }
 
+            // End of line
+            w.write_char(';')?;
+            w.new_line()?;
+
             i += 1;
         }
     }