Don't pass _count if _id is specified

[ir4y]

This code

client.resources("Patient").search(_id="12724066").get()

returns

OperationOutcome: {
  "resourceType": "OperationOutcome",
  "issue": [
    {
      "severity": "error",
      "code": "invalid",
      "details": {
        "text": "_count: unsupported when _id is provided"
      }
    }
  ]
}

for Cerner sandbox.

[ruscoder]

After #111 there's a workaround for this particular issue: client.reference("Patient", "12724066").to_resource()

But for conditional update it still might be an issue