matrix-sdk: Don't set two content-type headers for json contents

Ruma will for some requests already set the content-type for us to
application/json, but for some it still seems to miss the header, since
the headers are kept in a map add the header only if it isn't already
there.

matrix_sdk/src/http_client.rs

@@ -130,7 +130,7 @@ impl HttpClient {
             if let Some(content_type) = content_type {
                 request
                     .headers_mut()
-                    .append(http::header::CONTENT_TYPE, content_type);
+                    .insert(http::header::CONTENT_TYPE, content_type);
             }
         }