Prevent cyclic ref crash in JSON encode

Note that this commit makes no promise about great JSON output from the encoder, but the cyclic refs should be broken.

Fixes #1123

hugolib/siteJSONEncode_test.go

@@ -0,0 +1,32 @@
+package hugolib
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+)
+
+// Issue #1123
+// Testing prevention of cyclic refs in JSON encoding
+// May be smart to run with: -timeout 4000ms
+func TestEncodePage(t *testing.T) {
+
+	// borrowed from menu_test.go
+	s := createTestSite(MENU_PAGE_SOURCES)
+	testSiteSetup(s, t)
+
+	j, err := json.Marshal(s)
+	check(t, err)
+	fmt.Println("Site as JSON", string(j))
+
+	p, err := json.Marshal(s.Pages[0])
+	check(t, err)
+	fmt.Println("Page as JSON", string(p))
+
+}
+
+func check(t *testing.T, err error) {
+	if err != nil {
+		t.Fatalf("Failed %s", err)
+	}
+}