Mapper plugin overwrite date mapping
If you define some specific mapping for your file content, such as the following:

```javascript
{
    "person": {
        "properties": {
            "file": {
                "type": "attachment",
                "path": "full",
                "fields": {
                    "date": { "type": "string" }
                }
            }
        }
    }
}
```

And then, if you ask back the mapping, you get:

```javascript
{
   "person":{
      "properties":{
         "file":{
            "type":"attachment",
            "path":"full",
            "fields":{
               "file":{
                  "type":"string"
               },
               "author":{
                  "type":"string"
               },
               "title":{
                  "type":"string"
               },
               "name":{
                  "type":"string"
               },
               "date":{
                  "type":"date",
                  "format":"dateOptionalTime"
               },
               "keywords":{
                  "type":"string"
               },
               "content_type":{
                  "type":"string"
               }
            }
         }
      }
   }
}
```

All your settings have been overwrited by the mapper plugin.

See also issue #22 where the issue was found.

Closes #39.