Can't modify mapping if it has a parent

[kzwang]

If a type mapping with a parent is already exist, it can't be updated even if we are not modify the parent.

Seems related to #3849

I think that issue should be not allow to update the parent field in the mapping, however it will throw this exception even if we are updating other fields for mapping with parent

org.elasticsearch.index.mapper.MergeMappingException: Merge failed with failures {[The _parent field can't be added or updated]}" 

Example:

# create mapping for tweet
curl -XPUT localhost:9200/test/tweet/_mapping -d '{
    "tweet" : {
        "properties" : {
            "message" : {"type" : "string", "store" : "yes"}
        }
    }
}'

# create mapping for tweet2 with parent tweet
curl -XPUT localhost:9200/test/tweet2/_mapping -d '{
    "tweet2": {
        "_parent": {"type": "tweet"},
        "properties": {
            "message": {"type": "string", "store": "yes"}
        }
    }
}'

# add a new property to tweet2 mapping, it's not modifying the parent but still got exception
curl -XPUT localhost:9200/test/tweet2/_mapping -d '{
    "tweet2" : {
        "properties" : {
            "message2" : {"type" : "string", "store" : "yes"}
        }
    }
}'

[kzwang]

If the updated mapping has a "_parent" with same value it can be successfully merged