[SPARK-2888] [SQL] Fix addColumnMetadataToConf in HiveTableScan

JIRA: https://issues.apache.org/jira/browse/SPARK-2888

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1817 from yhuai/fixAddColumnMetadataToConf and squashes the following commits:

fba728c [Yin Huai] Fix addColumnMetadataToConf.
[SPARK-2908] [SQL] JsonRDD.nullTypeToStringType does not convert all NullType to StringType

JIRA: https://issues.apache.org/jira/browse/SPARK-2908

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1840 from yhuai/SPARK-2908 and squashes the following commits:

86e833e [Yin Huai] Update test.
cb11759 [Yin Huai] nullTypeToStringType should check columns with the type of array of structs.
[SPARK-2877] [SQL] MetastoreRelation should use SparkClassLoader when creating the tableDesc

JIRA: https://issues.apache.org/jira/browse/SPARK-2877

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1806 from yhuai/SPARK-2877 and squashes the following commits:

4142bcb [Yin Huai] Use Spark's classloader.