[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.