Agent doesn't proxy GET parameters on health check

[sgmiller]

Describe the bug
When issuing a request to /v1/sys/health with query parameters (such as standbyok), those parameters are not proxied to Vault.

This means we can't set up a health check in Kubernetes as it expects 200.

To Reproduce
Steps to reproduce the behavior:

1. curl http://localhost:8200/v1/sys/health?standbyok=true
2. Note the 429 response if the agent proxies to a standby node.

Expected behavior
Query parameters should be supported by the agent.

Environment:
All versions 1.1.2.
Linux x86-64
Vault server configuration file(s):