fix(alerts): custom unmarshal of channel configuration Headers and Payload fields

[sanderblue]

Resolves: #101

Synopsis

The New Relic v2 API endpoint for alert channels returns two different data types for configuration.payload and configuration.headers. If either of those fields is "empty" or left black when creating an alert channel in the UI, the API will return an empty string. But if headers and/or payload are included in an alert channel, the API will return an object, hence the need for a custom umarshal method for those fields.

Example responses:

Population headers and payload

{
  "type": "webhook",
  "configuration": {
    "headers": {                            
      "x-header-test": "header-value"  <-- notice we have an object here
    },                                                 
    "payload": {                                 
      "account_id": "$ACCOUNT_ID"   <-- notice we have an object here
    },
    "payload_type": "application/json",
    "base_url": "http://example.com"
  },
  // ...
}

Empty headers and payload

{
  "type": "webhook",
  "configuration": {
    "headers":  "",  <-- notice this is a string and NOT an object, so we have to guard for this
    "payload": "",   <-- notice this is a string and NOT an object, so we have to guard for this
    "payload_type": "application/json",
    "base_url": "http://example.com"
  },
  // ...
}

[jthurman42]

[ctrombley]

One tiny change and otherwise LGTM.