Adjust decode/encode url function to keep the key parameter

See: https://github.com/zikula-modules/Feeds/issues/4

Current quick & dirty fix:

```
<a href="{modurl fqurl=true forcelongurl=true modname='Feeds' type='user' func='updatecache' key=$modvars.Feeds.key}">{modurl fqurl=true forcelongurl=true modname='Feeds' type='user' func='updatecache' key=$modvars.Feeds.key}</a>
```
