Added private method for checking if user is linked with a given auth type.

Author: nlutsenko

Parse/Internal/User/PFUserPrivate.h

@@ -86,6 +86,9 @@ extern NSString *const PFUserCurrentUserKeychainItemName;
 // TODO: (nlutsenko) Add Documentation
 - (BFTask *)unlinkWithAuthTypeInBackground:(NSString *)authType;
 
+// TODO: (nlutsenko) Add Documentation
+- (BOOL)isLinkedWithAuthType:(NSString *)authType;
+
 ///--------------------------------------
 /// @name Authentication Providers (Private)
 ///--------------------------------------

Parse/PFAnonymousUtils.m

@@ -42,7 +42,7 @@ + (void)logInWithTarget:(id)target selector:(SEL)selector {
 ///--------------------------------------
 
 + (BOOL)isLinkedWithUser:(PFUser *)user {
-    return [user.linkedServiceNames containsObject:[[[self _authenticationProvider] class] authType]];
+    return [user isLinkedWithAuthType:[[[self _authenticationProvider] class] authType]];
 }
 
 ///--------------------------------------

Parse/PFUser.m

@@ -891,6 +891,15 @@ - (BFTask *)unlinkWithAuthTypeInBackground:(NSString *)authType {
     return save;
 }
 
+#pragma mark Linked
+
+- (BOOL)isLinkedWithAuthType:(NSString *)authType {
+    PFParameterAssert(authType, @"Authentication type can't be `nil`.");
+    @synchronized(self.lock) {
+        return [self.linkedServiceNames containsObject:authType];
+    }
+}
+
 #pragma mark Private
 
 + (void)_unregisterAuthenticationProvider:(id<PFAuthenticationProvider>)provider {