Use higher-precision value to normalize DateTimeInterface

[msmuenchen]

The normalizeValue function in ModelToElasticaAutoTransformer serializes a DateTimeInterface value into the format string ´c` aka ISO 8601.

This can lead to a loss of precision - the date_format of Elasticsearch is clearly capable of ingesting RFC3339 Extended (aka ISO 8601 with fractional seconds) by specifying date_time as format.