Fix issue where AuthResult callback would not be called on successful auto-upgrade (#519)

Author: morganchen12

FirebaseAuthUI/FUIAuth.m

@@ -296,7 +296,7 @@ - (void)autoUpgradeAccountWithProviderUI:(id<FUIAuthProvider>)providerUI
               };
               NSError *mergeError = [FUIAuthErrorUtils mergeConflictErrorWithUserInfo:userInfo
                                                                       underlyingError:error];
-              [self completeSignInWithResult:nil
+              [self completeSignInWithResult:authResult
                                        error:mergeError
                     presentingViewController:presentingViewController
                                     callback:callback];
@@ -336,6 +336,11 @@ - (void)autoUpgradeAccountWithProviderUI:(id<FUIAuthProvider>)providerUI
           callback(nil, error);
         }
       }
+    } else {
+      [self completeSignInWithResult:authResult
+                               error:nil
+            presentingViewController:presentingViewController
+                            callback:callback];
     }
   }];
 }

FirebaseAuthUI/FUIAuthErrorUtils.m

@@ -32,6 +32,8 @@ + (NSError *)mergeConflictErrorWithUserInfo:(NSDictionary *)userInfo
   if (underlyingError != nil) {
     errorInfo[NSUnderlyingErrorKey] = underlyingError;
   }
+  errorInfo[NSLocalizedDescriptionKey] = @"Unable to merge accounts. Check the userInfo dictionary"
+      @" for the auth credential of the logged-in account.";
   return [self errorWithCode:FUIAuthErrorCodeMergeConflict userInfo:[errorInfo copy]];
 }