Variable shadowing occurs when variable name's casing is changed

[tonglil]

Describe the bug
It is possible to shadow the available vars by changing the casing of the vars: variable.

To reproduce

---
kind: pipeline
type: docker
name: default

steps:
  - name: gke
    image: nytimes/drone-gke
    pull: always
    environment:
      TOKEN:
        from_secret: GOOGLE_CREDENTIALS
    settings:
      dry_run: true
      verbose: true
      zone: us-east1-b
      cluster: test
      namespace: test
      vars:
        NAMESPACE: "override"

Proceeds successfully:

---START VARIABLES AVAILABLE FOR ALL TEMPLATES---
{
	"BRANCH": "test",
	"BUILD_NUMBER": "78",
	"COMMIT": "xxx",
	"NAMESPACE": "override",
	"TAG": "",
	"cluster": "test",
	"namespace": "test",
	"project": "my-project-x",
	"zone": "us-east1-b"
}
---END VARIABLES AVAILABLE FOR ALL TEMPLATES---

Expected behavior
Should error out with Error: var "namespace" shadows existing var.

Version (please complete the following information):

• Drone: 1.9.0
• drone-gke: latest
• Kubernetes:

Client Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.9", GitCommit:"2e808b7cb054ee242b68e62455323aa783991f03", GitTreeState:"clean", BuildDate:"2020-01-18T23:33:14Z", GoVersion:"go1.12.12", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"15+", GitVersion:"v1.15.12-gke.16", GitCommit:"013d168bdbdca7bc365fae800272aede20848d7e", GitTreeState:"clean", BuildDate:"2020-07-29T03:47:52Z", GoVersion:"go1.12.17b4", Compiler:"gc", Platform:"linux/amd64"}

Additional context

Potential fix: lowercase all variables when comparing for shadowing.

https://github.com/nytimes/drone-gke/blob/master/main.go#L430
https://github.com/nytimes/drone-gke/blob/master/main.go#L514
https://github.com/nytimes/drone-gke/blob/master/main.go#L532