static boolean needs100Continue(RequestHeaders headers) {
        return headers.contains(HttpHeaderNames.EXPECT, HttpHeaderValues.CONTINUE.toString());
    }
Writes the {@link RequestHeaders} to the {@link Channel}.
The {@link RequestHeaders} is merged with {@link ClientRequestContext#additionalRequestHeaders()}
before being written.
Note that the written data is not flushed by this method. The caller should explicitly call
{@link Channel#flush()} when each write unit is done.
core/src/main/java/com/linecorp/armeria/client/AbstractHttpRequestHandler.java