MXRestClient: Improve registration parameters handling.

The device display name is now provided by the Riot application.

Related to `Device name leaks personal information` element-hq/element-ios#910

MatrixSDK/MXRestClient.h

@@ -203,6 +203,10 @@ typedef enum : NSUInteger
 
  At the end of the registration process, the SDK user should be able to construct a MXCredentials object
  from the response of the last registration action request.
+ 
+ @note The caller may provide the device display name by adding @"initial_device_display_name" key
+ in the `parameters` dictionary. If the caller does not provide it, the device display name field
+ is filled with the device name.
 
  @param parameters the parameters required for the current registration stage
  @param success A block object called when the operation succeeds. It provides the raw JSON response
@@ -284,7 +288,11 @@ typedef enum : NSUInteger
 
  @see the register method for explanation of flows that require to make several request to the
  home server.
-
+ 
+ @note The caller may provide the device display name by adding @"initial_device_display_name" key
+ in the `parameters` dictionary. If the caller does not provide it, the device display name field
+ is filled with the device name.
+ 
  @param parameters the parameters required for the current login stage
  @param success A block object called when the operation succeeds. It provides the raw JSON response
  from the server.
@@ -300,6 +308,8 @@ typedef enum : NSUInteger
  Log a user in.
 
  This method manages the full flow for simple login types and returns the credentials of the logged matrix user.
+ 
+ @note The device display name field is filled with the device name by default.
 
  @param loginType the login type. Only kMXLoginFlowTypePassword (m.login.password) is supported.
  @param username the user id (ex: "@bob:matrix.org") or the user id localpart (ex: "bob") of the user to register.

MatrixSDK/MXRestClient.m

@@ -796,23 +796,36 @@ - (MXHTTPOperation*)registerOrLogin:(MXAuthAction)authAction parameters:(NSDicti
 {
     // If the caller does not provide it, fill the device display name field with the device name
     // Do it only if parameters contains the password field, do make homeserver happy.
-    if (parameters[@"password"] && !parameters[@"initial_device_display_name"])
+    if (parameters[@"password"])
     {
-        NSMutableDictionary *newParameters = [NSMutableDictionary dictionaryWithDictionary:parameters];
+        NSMutableDictionary *newParameters;
+
+        if (!parameters[@"initial_device_display_name"])
+        {
+            newParameters = [NSMutableDictionary dictionaryWithDictionary:parameters];
+
 #if TARGET_OS_IPHONE
-        NSString *deviceName = [UIDevice currentDevice].name;
+            NSString *deviceName = [UIDevice currentDevice].name;
 #elif TARGET_OS_OSX
-        NSString *deviceName = [NSHost currentHost].localizedName;
+            NSString *deviceName = [NSHost currentHost].localizedName;
 #endif
-        newParameters[@"initial_device_display_name"] = deviceName;
+            newParameters[@"initial_device_display_name"] = deviceName;
+        }
 
         if (MXAuthActionRegister == authAction)
         {
             // Patch: Add the temporary `x_show_msisdn` flag to not filter the msisdn login type in the supported authentication flows.
+            if (!newParameters)
+            {
+                newParameters = [NSMutableDictionary dictionaryWithDictionary:parameters];
+            }
             newParameters[@"x_show_msisdn"] = @(YES);
         }
 
-        parameters = newParameters;
+        if (newParameters)
+        {
+            parameters = newParameters;
+        }
     }
 
     return [httpClient requestWithMethod:@"POST"