Warnings unserializable

**Description**
Storing sampling warnings (in `SampleSet.info`) renders sampleset JSON unserializable.

**To Reproduce**
```python
>>> import json
>>> import dimod
>>> from dwave.system import EmbeddingComposite, DWaveSampler
>>> from dwave.system.warnings import WarningAction
>>>
>>> bqm = dimod.BQM.from_ising({},{'ab': 1, 'bc': 1, 'ca': 1})
>>> sampler = EmbeddingComposite(DWaveSampler())
>>>
>>> sampleset = sampler.sample(bqm, return_embedding=True, warnings=WarningAction.SAVE)
>>>
>>> json.dumps(sampleset.to_serializable())
Traceback (most recent call last):
File "", line 1, in
...
TypeError: Object of type type is not JSON serializable
```

**Expected behavior**
`json.dumps(sampleset.to_serializable())` should work.