By default when {@link #getMessageHeaders()} is called, {@code "this"}
{@code MessageHeaderAccessor} instance can no longer be used to modify the
underlying message headers. However if {@link #setLeaveMutable()}
is used, this method is necessary to indicate explicitly when the
{@code MessageHeaders} instance should no longer be modified.
@since 4.1
By default when {@link #getMessageHeaders()} is called, {@code "this"} {@code MessageHeaderAccessor} instance can no longer be used to modify the underlying message headers. However if {@link #setLeaveMutable()} is used, this method is necessary to indicate explicitly when the {@code MessageHeaders} instance should no longer be modified. @since 4.1