protocol: mark IncompleteMessageType as Copy

warning: this argument is passed by value, but not consumed in the function body
  --> src/protocol/message.rs:98:30
   |
98 |     pub fn new(message_type: IncompleteMessageType) -> Self {
   |                              ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(needless_pass_by_value)] on by default
help: consider marking this type as Copy
  --> src/protocol/message.rs:135:1
   |
135| / pub enum IncompleteMessageType {
136| |     Text,
137| |     Binary,
138| | }
   | |_^
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#needless_pass_by_value

src/protocol/message.rs

@@ -159,6 +159,7 @@ impl IncompleteMessage {
 }
 
 /// The type of incomplete message.
+#[derive(Copy, Clone)]
 pub enum IncompleteMessageType {
     Text,
     Binary,