Fix sychronization and traversal issues in ADCustomHeaderHandler
Fix sychronization and traversal issues in ADCustomHeaderHandler

Ran across these where going over the code with @unpluggedk. Currently we're mutating an array while traversing it, which is generally a bad thing to do. There also is no protection here if something modifies the array while we're traversing it, so fixing both issues.

AzureAD/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc,AzureAD/azure-activedirectory-library-for-objc