MarshalIndent 方法内漏掉了对 extensions 的拷贝

[yelo-17133]

func (cfg *frozenConfig) MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {

newCfg := cfg.configBeforeFrozen
newCfg.IndentionStep = len(indent)
return newCfg.frozeWithCacheReuse().Marshal(v)

to：
newCfg := cfg.configBeforeFrozen
newCfg.IndentionStep = len(indent)
newFrozenCfg := newCfg.frozeWithCacheReuse();
newFrozenCfg.extensions = cfg.extensions
return newFrozenCfg.Marshal(v)