Return the underlying {@code MessageHeaders} instance.
<p>Unless {@link #setLeaveMutable()} was set to {@code true}, after
this call, the headers are immutable and this accessor can no longer
modify them.
<p>This method always returns the same {@code MessageHeaders} instance if
invoked multiples times. To obtain a copy of the underlying headers, use
{@link #toMessageHeaders()} or {@link #toMap()} instead.
@since 4.1
Return the underlying {@code MessageHeaders} instance. <p>Unless {@link #setLeaveMutable()} was set to {@code true}, after this call, the headers are immutable and this accessor can no longer modify them. <p>This method always returns the same {@code MessageHeaders} instance if invoked multiples times. To obtain a copy of the underlying headers, use {@link #toMessageHeaders()} or {@link #toMap()} instead. @since 4.1