Cannot concatenate sample sets of unlike types

minimal failing example:
```
import dimod
import numpy as np

sample = [-1, 1, 1]

ss_int = dimod.SampleSet.from_samples(np.asarray(sample, dtype=np.int), energy=6, vartype='SPIN')
ss_float = dimod.SampleSet.from_samples(np.asarray(sample, dtype=np.float), energy=6, vartype='SPIN')

dimod.sampleset.concatenate([ss_int, ss_float])
```