allow non-'static HTTP method names

Background: RTSP uses HTTP digest authentication (see RFC 2326 section
1.4). I'm using the rtsp-types crate. It works fine with digest_auth,
except that [rtsp_types::Method::Extension](https://docs.rs/rtsp-types/0.0.1/rtsp_types/enum.Method.html#variant.Extension)
has a non-static lifetime.

src/digest.rs

@@ -161,7 +161,7 @@ pub struct AuthContext<'a> {
     /// May be left out if not using auth-int
     pub body: Option<Cow<'a, [u8]>>,
     /// HTTP method used (defaults to GET)
-    pub method: HttpMethod,
+    pub method: HttpMethod<'a>,
     /// Spoofed client nonce (use only for tests; a random nonce is generated automatically)
     pub cnonce: Option<Cow<'a, str>>,
 }
@@ -202,7 +202,7 @@ impl<'a> AuthContext<'a> {
         password: PW,
         uri: UR,
         body: Option<BD>,
-        method: HttpMethod,
+        method: HttpMethod<'a>,
     ) -> Self
     where
         UN: Into<Cow<'a, str>>,

src/enums.rs

@@ -168,20 +168,20 @@ impl Display for Charset {
 
 /// HTTP method (used when generating the response hash for some Qop options)
 #[derive(Debug)]
-pub enum HttpMethod {
+pub enum HttpMethod<'a> {
     GET,
     POST,
     HEAD,
-    OTHER(&'static str),
+    OTHER(&'a str),
 }
 
-impl Default for HttpMethod {
+impl<'a> Default for HttpMethod<'a> {
     fn default() -> Self {
         HttpMethod::GET
     }
 }
 
-impl Display for HttpMethod {
+impl<'a> Display for HttpMethod<'a> {
     /// Convert to uppercase string
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         f.write_str(match self {