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ARROW-393: [JAVA] JSON file reader fails to set the buffer size on String data vector

Fixed by calling setValueCount after setting the values instead of before.
Since we set the inner vectors of NullableVarCharVector directly we don't have to worry about it's lastSet field and the way null values are handled.

Author: Julien Le Dem <julien@dremio.com>

Closes #218 from julienledem/json_read_varchar and squashes the following commits:

e147906 [Julien Le Dem] ARROW-393: [JAVA] JSON file reader fails to set the buffer size on String data vector