Cassandra fails to deploy with persistence disabled

[touchdown]

Is this a request for help?:
No

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

Version of Helm and Kubernetes:
Helm v2.4.2
K8s v1.6.4

Which chart:
incubator/cassandra

What happened:
setting persistence.enabled = false doesn't deploy

What you expected to happen:
deploy cassandra cluster with emptyDir

How to reproduce it (as minimally and precisely as possible):
helm install --namespace "cassandra" -n "cassandra" --set persistence.enabled=false incubator/cassandra

Error: error validating "": error validating data: [field spec.template.spec.containers[0].volumeMounts[0].mountPath for v1.VolumeMount is required, found invalid field emptyDir for v1.VolumeMount]

Anything else we need to know:
I have a fix locally, just want to make sure it is ok to submit a PR for the fix @KongZ

[KongZ]

@touchdown Sure, please submit a PR. Thanks a lot :)