net/url: document and add example for ParseQuery("x")

Fixes #16460.

Change-Id: Ie9d5f725d2d7e8210ab6f7604a5a05fc49f707de
Reviewed-on: https://go-review.googlesource.com/31331
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/example_test.go

@@ -5,6 +5,7 @@
 package url_test
 
 import (
+	"encoding/json"
 	"fmt"
 	"log"
 	"net/http"
@@ -98,3 +99,21 @@ func ExampleURL_ResolveReference() {
 	// Output:
 	// http://example.com/search?q=dotnet
 }
+
+func ExampleParseQuery() {
+	m, err := url.ParseQuery(`x=1&y=2&y=3;z`)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(toJSON(m))
+	// Output:
+	// {"x":["1"], "y":["2", "3"], "z":[""]}
+}
+
+func toJSON(m interface{}) string {
+	js, err := json.Marshal(m)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return strings.Replace(string(js), ",", ", ", -1)
+}

src/net/url/url.go

@@ -777,6 +777,10 @@ func (v Values) Del(key string) {
 // ParseQuery always returns a non-nil map containing all the
 // valid query parameters found; err describes the first decoding error
 // encountered, if any.
+//
+// Query is expected to be a list of key=value settings separated by
+// ampersands or semicolons. A setting without an equals sign is
+// interpreted as a key set to an empty value.
 func ParseQuery(query string) (Values, error) {
 	m := make(Values)
 	err := parseQuery(m, query)