http api /v1/kv/ should check true value for the parameter "recurse"

#### Overview of the Issue

According to [consul doc on kv API](https://www.consul.io/api-docs/kv),

> If the [recurse](https://www.consul.io/api-docs/kv#recurse) or [keys](https://www.consul.io/api-docs/kv#keys) query parameters are true, this endpoint will return an array of keys

However, the implementation does recurse if `recurse` exists.

https://github.com/hashicorp/consul/blob/474385d153e21203c190d1292e039af0852ee5b0/agent/kvs_endpoint.go#L56-L57

Should check the value of the parameter instead of its existence.