Outlaw nil keys in job parameters. Coerce all values to strings

This change coerces all parameter values to strings before serialisation into Redis.

e.g. 
```
{'key1' => 'value', 'key2' => 1, 'key3' => true}
```
is serialised into JSON as:
```
{"key1":"value","key2":"1","key3":"true"}
```
`maps`, `arrays` and other complex objects are serialised using their default `to_s` method but usage is discouraged.