Reserved aggregation names

NEST uses a heuristics based parser to parse the aggregations back in to typed responses. Because of this, some of the key properties we use to make decisions about parsing are not allowed as key names for aggregations in a request.

This limitation will be lifted but at some point in the future, but for the meantime, avoid the following names for aggregation keys:

new []
{
    "score",
    "value_as_string",
    "keys",
    "max_score"
}