Fixes broken HTTP header and method for health checks.

[slackpad]

These didn't get plumbed through so were not getting applied. Also reordered them to match the order in the CheckType structure.

[slackpad]

With this config:

$ cat header.json
{
  "check": {
    "id": "api",
    "name": "HTTP API on port 5000",
    "http": "http://localhost:5000/health",
    "method": "POST",
    "header": {"x-foo":["bar", "baz"]},
    "interval": "10s",
    "timeout": "1s"
  }
}

Before the fix:

$ nc -l 5000
GET /health HTTP/1.1
Host: localhost:5000
User-Agent: Consul Health Check
Accept: text/plain, text/*, */*
Accept-Encoding: gzip
Connection: close

After the fix:

$ nc -l 5000
POST /health HTTP/1.1
Host: localhost:5000
User-Agent: Consul Health Check
Content-Length: 0
Accept: text/plain, text/*, */*
x-foo: bar
x-foo: baz
Accept-Encoding: gzip
Connection: close

[slackpad]

This is kind of a temporary fix, so opened #3179 to track getting rid of the multiple structures so we don't hit this issue again.