Setting http.DefaultClient in store/consul/consul.go causes failure in consul API 0.8.1, Go 1.8.1

[ebudan]

Hello

When working against version 0.8.1 of consul-api under Go 1.8.1, the following line causes consul-api to fail (interface conversion of nil): https://github.com/docker/libkv/blob/master/store/consul/consul.go#L75

Commenting this line out lets consul-api proceed with its own default client initialization, which would seem to be enough for the purposes here. (I have't verified that TLS will work, however.)