pkg/tool/http: make request.body optional

For simple GET requests this is quite weird to have to set:

`request: body: ""`

Change-Id: I41a86ba5a5ab2dd25dad792e2efa4b95b874e30f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9722
Reviewed-by: CUE cueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: eonpatapon

pkg/tool/http/doc.go

@@ -26,7 +26,7 @@ Do: {
 	url:    string // TODO: make url.URL type
 
 	request: {
-		body: *bytes | string
+		body?: bytes | string
 		header: [string]:  string | [...string]
 		trailer: [string]: string | [...string]
 	}

pkg/tool/http/http.cue

@@ -18,6 +18,7 @@ package http
 //go:generate gofmt -s -w .
 
 import (
+	"bytes"
 	"io"
 	"io/ioutil"
 	"net/http"
@@ -52,6 +53,8 @@ func (c *httpCmd) Run(ctx *task.Context) (res interface{}, err error) {
 			if err != nil {
 				return nil, err
 			}
+		} else {
+			r = bytes.NewReader([]byte(""))
 		}
 		if header, err = parseHeaders(obj, "header"); err != nil {
 			return nil, err