chore(lib): fix new unused_mut warnings in nightly

src/header/common/authorization.rs

@@ -170,7 +170,7 @@ impl FromStr for Basic {
         match decode(s) {
             Ok(decoded) => match String::from_utf8(decoded) {
                 Ok(text) => {
-                    let mut parts = &mut text.split(':');
+                    let parts = &mut text.split(':');
                     let user = match parts.next() {
                         Some(part) => part.to_owned(),
                         None => return Err(::Error::Header)

src/header/common/link.rs

@@ -495,7 +495,7 @@ impl FromStr for Link {
                     Some(p) => p.trim(),
                 };
 
-                let mut link_header = match link_values.last_mut() {
+                let link_header = match link_values.last_mut() {
                     None => return Err(::Error::Header),
                     Some(l) => l,
                 };