assign where with operator contains and a "&" in value fails

When using the operator "contains" of an assign where statement, the use of the "&" character within the value is not possible.

Following error occurs:
```
array_key_exists(): The first argument should be either a string or an integer (IcingaObject.php:184)
```

## Your Environment
* Director version (System - About): 1.10.2
* Icinga Web 2 version and modules (System - About): 2.11.2

