public Builder setExpectContinueEnabled(final boolean expectContinueEnabled) {
            this.expectContinueEnabled = expectContinueEnabled;
            return this;
        }
Determines whether the 'Expect: 100-Continue' handshake is enabled
for entity enclosing methods. The purpose of the 'Expect: 100-Continue'
handshake is to allow a client that is sending a request message with
a request body to determine if the origin server is willing to
accept the request (based on the request headers) before the client
sends the request body.
<p>
The use of the 'Expect: 100-continue' handshake can result in
a noticeable performance improvement for entity enclosing requests
(such as POST and PUT) that require the target server's
authentication.
</p>
<p>
'Expect: 100-continue' handshake should be used with caution, as it
may cause problems with HTTP servers and proxies that do not support
HTTP/1.1 protocol.
</p>
<p>
Default: {@code false}
</p>

@return this instance.
httpclient5/src/main/java/org/apache/hc/client5/http/config/RequestConfig.java