default RequestHeadersBuilder scheme(SessionProtocol sessionProtocol) {
        requireNonNull(sessionProtocol, "sessionProtocol");
        scheme(HttpHeadersUtil.getScheme(sessionProtocol));
        return this;
    }
Sets the {@code ":scheme"} header from the specified {@link SessionProtocol}.
core/src/main/java/com/linecorp/armeria/common/RequestHeadersBuilder.java