protected AbstractRequestContextBuilder method(HttpMethod method) {
        requireNonNull(method, "method");
        if (req != null) {
            checkArgument(method == req.method(),
                          "method: %s (expected: same as request.method)", method);
        } else {
            this.method = method;
        }
        return this;
    }
Sets the {@link HttpMethod} of the request.

@throws IllegalArgumentException if the specified {@link HttpMethod} is not same with the
                                 {@link HttpMethod} of the {@link HttpRequest} you specified when
                                 creating this builder. This exception is not thrown if you
                                 created a builder with an {@link RpcRequest}.
core/src/main/java/com/linecorp/armeria/common/AbstractRequestContextBuilder.java