When a field is not mapped, Elasticsearch tries to generate a mapping update
from the parsed document. Some documents can introduce corner-cases, for
instance in the event of a multi-valued field whose values would be mapped to
different field types if they were supplied on their own, see for instance:

```
PUT index/doc/1
{
  "foo": ["2017-11-10T02:00:01.247Z","bar"]
}
```

In that case, dynamic mappings want to map the first value as a `date` field
and the second one as a `text` field. This currently throws an exception,
which is expected, but the wrong one since it throws a `class_cast_exception`
(which triggers a HTTP 5xx code) when it should throw an
`illegal_argument_exception` (HTTP 4xx).

When a field is not mapped, Elasticsearch tries to generate a mapping update
from the parsed document. Some documents can introduce corner-cases, for
instance in the event of a multi-valued field whose values would be mapped to
different field types if they were supplied on their own, see for instance:

```
PUT index/doc/1
{
  "foo": ["2017-11-10T02:00:01.247Z","bar"]
}
```

In that case, dynamic mappings want to map the first value as a `date` field
and the second one as a `text` field. This currently throws an exception,
which is expected, but the wrong one since it throws a `class_cast_exception`
(which triggers a HTTP 5xx code) when it should throw an
`illegal_argument_exception` (HTTP 4xx).

When a field is not mapped, Elasticsearch tries to generate a mapping update
from the parsed document. Some documents can introduce corner-cases, for
instance in the event of a multi-valued field whose values would be mapped to
different field types if they were supplied on their own, see for instance:

```
PUT index/doc/1
{
  "foo": ["2017-11-10T02:00:01.247Z","bar"]
}
```

In that case, dynamic mappings want to map the first value as a `date` field
and the second one as a `text` field. This currently throws an exception,
which is expected, but the wrong one since it throws a `class_cast_exception`
(which triggers a HTTP 5xx code) when it should throw an
`illegal_argument_exception` (HTTP 4xx).

* master: (41 commits)
  [Test] Fix AggregationsTests#testFromXContentWithRandomFields
  [DOC] Fix mathematical representation on interval (range) (elastic#27450)
  Update version check for CCS optional remote clusters
  Bump BWC version to 6.1.0 for elastic#27469
  Adapt rest test BWC version after backport
  Fix dynamic mapping update generation. (elastic#27467)
  Use the primary_term field to identify parent documents (elastic#27469)
  Move composite aggregation to core (elastic#27474)
  Fix test BWC version after backport
  Protect shard splitting from illegal target shards (elastic#27468)
  Cross Cluster Search: make remote clusters optional (elastic#27182)
  [Docs] Fix broken bulleted lists (elastic#27470)
  Move resync request serialization assertion
  Fix resync request serialization
  Fix issue where pages aren't released (elastic#27459)
  Add YAML REST tests for filters bucket agg (elastic#27128)
  Remove tcp profile from low level nio channel (elastic#27441)
  [TEST] Fix `GeoShapeQueryTests#testPointsOnly` failure
  Transition transport apis to use void listeners (elastic#27440)
  AwaitsFix GeoShapeQueryTests#testPointsOnly elastic#27454
  ...