[SPARK-2375][SQL] JSON schema inference may not resolve type conflicts correctly for a field inside an array of structs

For example, for
```
{"array": [{"field":214748364700}, {"field":1}]}
```
the type of field is resolved as IntType. While, for
```
{"array": [{"field":1}, {"field":214748364700}]}
```
the type of field is resolved as LongType.

JIRA: https://issues.apache.org/jira/browse/SPARK-2375

Author: Yin Huai <huaiyin.thu@gmail.com>

Closes #1308 from yhuai/SPARK-2375 and squashes the following commits:

3e2e312 [Yin Huai] Update unit test.
1b2ff9f [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2375
10794eb [Yin Huai] Correctly resolve the type of a field inside an array of structs.