Validate JSON snippets are not only expected, but valid. (uber-go#146)

Inspired by the review of uber-go#145

Author: billf

field_test.go

@@ -21,6 +21,7 @@
 package zap
 
 import (
+	"encoding/json"
 	"errors"
 	"net"
 	"strings"
@@ -46,6 +47,11 @@ func assertFieldJSON(t testing.TB, expected string, field Field) {
 	enc := newJSONEncoder()
 	defer enc.Free()
 
+	var out interface{}
+	err := json.Unmarshal([]byte("{"+expected+"}"), &out)
+	require.NoError(t, err,
+		"Expected JSON snippet %q must be valid for use in an object.", expected)
+
 	field.AddTo(enc)
 	assert.Equal(t, expected, string(enc.bytes),
 		"Unexpected JSON output after applying field %+v.", field)