Give rejection names a finite lifetime

In case the way headers are handled is changed at a later date, this
change prevents commitments to how long returned references to rejection
related names should live.

Rather than the static lifetime these references will now live for as
long rejections themselves.

src/reject.rs

@@ -506,9 +506,9 @@ pub struct MissingHeader {
     name: &'static str,
 }
 
-impl MissingHeader {
+impl<'a> MissingHeader {
     /// Retrieve the name of the header that was missing
-    pub fn name(self) -> &'static str {
+    pub fn name(self) -> &'a str {
         self.name
     }
 }
@@ -527,9 +527,9 @@ pub struct InvalidHeader {
     name: &'static str,
 }
 
-impl InvalidHeader {
+impl<'a> InvalidHeader {
     /// Retrieve the name of the header that was invalid
-    pub fn name(self) -> &'static str {
+    pub fn name(self) -> &'a str {
         self.name
     }
 }
@@ -548,9 +548,9 @@ pub struct MissingCookie {
     name: &'static str,
 }
 
-impl MissingCookie {
+impl<'a> MissingCookie {
     /// Retrieve the name of the cookie that was missing
-    pub fn name(self) -> &'static str {
+    pub fn name(self) -> &'a str {
         self.name
     }
 }