MergeRequestEventPayload has improper unmarshalling for changes

[EmilLuta]

By default, the changes field of the JSON will look something like this (according to: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#merge-request-events):

...
"changes":
    "author_id": {
        "previous": null,
        "current": 1
    },
    "merge_params": {
        "previous": {
        },
        "current": {
            "force_remove_source_branch": "0"
        }
    },...
...

The current format looks as it follows:

type Changes struct {
	LabelChanges LabelChanges `json:"labels"`
}

// LabelChanges contains changes in labels associated with a GitLab issue or MR
type LabelChanges struct {
	Previous []Label `json:"previous"`
	Current  []Label `json:"current"`
}

// Label contains all of the GitLab label information
type Label struct {
	ID          int64      `json:"id"`
	Title       string     `json:"title"`
	Color       string     `json:"color"`
	ProjectID   int64      `json:"project_id"`
	CreatedAt   customTime `json:"created_at"`
	UpdatedAt   customTime `json:"updated_at"`
	Template    bool       `json:"template"`
	Description string     `json:"description"`
	Type        string     `json:"type"`
	GroupID     int64      `json:"group_id"`
}

I propose that this should be marked as (excuse my poor Golang):

type Changes map[string]Change

type Change struct {
    Current  interface{}
    Previous interface{}
}

Unless objections, I'll be providing a PR for this.

Thanks in advance!