frame: change CloseCode::is_allowed to by-value

It's a small copy type so copying is more conventional.

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> src/protocol/frame/coding.rs:186:23
    |
186 |     pub fn is_allowed(&self) -> bool {
    |                       ^^^^^ help: consider passing by value instead: `self`
    |
    = note: #[warn(trivially_copy_pass_by_ref)] on by default
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

This is a breaking change.

src/protocol/frame/coding.rs

@@ -183,8 +183,8 @@ pub enum CloseCode {
 
 impl CloseCode {
     /// Check if this CloseCode is allowed.
-    pub fn is_allowed(&self) -> bool {
-        match *self {
+    pub fn is_allowed(self) -> bool {
+        match self {
             Bad(_)      => false,
             Reserved(_) => false,
             Status      => false,