micro_http: check arithmetic operations

Signed-off-by: Ioana Chirca <chioana@amazon.com>
Signed-off-by: YUAN LYU <lyuyuan92@gmail.com>

src/response.rs

@@ -109,6 +109,7 @@ impl ResponseHeaders {
         let delimitator = b", ";
         for (idx, method) in self.allow.iter().enumerate() {
             buf.write_all(method.raw())?;
+            // We check above that `self.allow` is not empty.
             if idx < self.allow.len() - 1 {
                 buf.write_all(delimitator)?;
             }