Releases the current 'run as' (assumed) identity and reverts back to the previous 'pre run as'
identity that existed before {@code #runAs runAs} was called.
<p/>
This method returns 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not
operating under an assumed identity.
@return the 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not operating
under an assumed identity.
@see #runAs
Releases the current 'run as' (assumed) identity and reverts back to the previous 'pre run as' identity that existed before {@code #runAs runAs} was called. <p/> This method returns 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not operating under an assumed identity.
@return the 'run as' (assumed) identity being released or {@code null} if this {@code Subject} is not operating under an assumed identity. @see #runAs