[fractal] fractal-matrix-api: Don't call a function in Result::unwrap() arg

[Alexandre Franke <afranke src gnome org>]

commit 5295964b6a7d936e9ed65f68f1e6b5d369fa2c94
Author: Zeeshan Ali <zeenix collabora co uk>
Date:   Wed Dec 12 23:05:44 2018 +0100

    fractal-matrix-api: Don't call a function in Result::unwrap() arg
    
    This fixes this errors from clippy:
    
    ```
    warning: use of `unwrap_or` followed by a call to `new`
      --> fractal-matrix-api/src/backend/room.rs:80:30
       |
    80 |                       avatar = util::get_room_avatar(&baseu, &tk, &userid, &roomid)
       |  ______________________________^
    81 | |                         .unwrap_or(String::from(""));
       | |_________________________________________________^ help: try this: `util::get_room_avatar(&baseu, 
&tk, &userid, &roomid).unwrap_or_default()`
       |
       = note: #[warn(clippy::or_fun_call)] on by default
       = help: for further information visit 
https://rust-lang-nursery.github.io/rust-clippy/master/index.html#or_fun_call
    ```
    
    Related: #370

 fractal-matrix-api/src/backend/room.rs | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)
---
diff --git a/fractal-matrix-api/src/backend/room.rs b/fractal-matrix-api/src/backend/room.rs
index 09f8898..2f33384 100644
--- a/fractal-matrix-api/src/backend/room.rs
+++ b/fractal-matrix-api/src/backend/room.rs
@@ -74,16 +74,14 @@ pub fn get_room_avatar(bk: &Backend, roomid: String) -> Result<(), Error> {
                         String::from("")
                     }
                 }
-                None => {
-                    util::get_room_avatar(&baseu, &tk, &userid, &roomid).unwrap_or(String::from(""))
-                }
+                None => util::get_room_avatar(&baseu, &tk, &userid, &roomid).unwrap_or_default(),
             };
             tx.send(BKResponse::RoomAvatar(roomid, avatar)).unwrap();
         },
         |err: Error| match err {
             Error::MatrixError(ref js) if js["errcode"].as_str().unwrap_or("") == "M_NOT_FOUND" => {
-                let avatar = util::get_room_avatar(&baseu, &tk, &userid, &roomid)
-                    .unwrap_or(String::from(""));
+                let avatar =
+                    util::get_room_avatar(&baseu, &tk, &userid, &roomid).unwrap_or_default();
                 tx.send(BKResponse::RoomAvatar(roomid, avatar)).unwrap();
             }
             _ => {
@@ -155,7 +153,7 @@ pub fn get_room_messages_from_msg(bk: &Backend, roomid: String, msg: Message) ->
     // normal get_room_messages
     let baseu = bk.get_base_url()?;
     let tk = bk.data.lock().unwrap().access_token.clone();
-    let id = msg.id.unwrap_or("".to_string());
+    let id = msg.id.unwrap_or_default();
     let tx = bk.internal_tx.clone();
 
     thread::spawn(move || {