Issue description: In a site using the multiplexerproviders for both role and membership, tagging users with @ does not list Active Directory (AD) users when trying to add comment to a page. The function works well for users from SqlServerMembershipProvider or WindowsMembershipProvider. The tagging feature is also work well when configure ActiveDirectoryMembershipProvider and ActiveDirectoryRoleProvider for site only, AD users are suggested with @ in comments.
<providers>
<clear />
<add name="MultiplexingMembershipProvider" type="EPiServer.Security.MultiplexingMembershipProvider, EPiServer.Framework.AspNet"
provider1="SqlServerMembershipProvider"
provider2="ActiveDirectoryMembershipProvider"
provider3="WindowsMembershipProvider" />
<add name="SqlServerMembershipProvider" type="System.Web.Providers.DefaultMembershipProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
connectionStringName="EPiServerDB" enablePasswordRetrieval="false"
enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false"
maxInvalidPasswordAttempts="5" minRequiredPasswordLength="6" minRequiredNonalphanumericCharacters="0"
passwordAttemptWindow="10" applicationName="/" />
<add name="WindowsMembershipProvider" type="EPiServer.Security.WindowsMembershipProvider, EPiServer.Cms.AspNet" deletePrefix="BUILTIN\" searchByEmail="true"/>
<add name="ActiveDirectoryMembershipProvider" maxInvalidPasswordAttempts="3" type="System.Web.Security.ActiveDirectoryMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
connectionStringName="ActiveDirectoryProviderConnection"
connectionUsername="########" connectionPassword="#########"
enableSearchMethods="true" attributeMapUsername="sAMAccountName" />
</providers>
</membership>
<roleManager enabled="true" defaultProvider="MultiplexingRoleProvider" cacheRolesInCookie="true">
<providers>
<clear />
<add name="MultiplexingRoleProvider" type="EPiServer.Security.MultiplexingRoleProvider, EPiServer.Framework.AspNet"
provider1="SqlServerRoleProvider"
provider2="ActiveDirectoryRoleProvider"
provider3="WindowsRoleProvider"
providerMap1="SqlServerMembershipProvider"
providerMap2="ActiveDirectoryMembershipProvider"
providerMap3="WindowsMembershipProvider" />
<add name="SqlServerRoleProvider" type="System.Web.Providers.DefaultRoleProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" connectionStringName="EPiServerDB" applicationName="/" />
<add name="ActiveDirectoryRoleProvider" type="EPiServer.Security.ActiveDirectoryRoleProvider, EPiServer.Cms.AspNet"
connectionStringName="ActiveDirectoryProviderConnection"
connectionUsername="#######" connectionPassword="#######" attributeMapUsername="sAMAccountName" />
<add name="WindowsRoleProvider" applicationName="/" type="EPiServer.Security.WindowsRoleProvider, EPiServer.Cms.AspNet"/>
</providers>
</roleManager>