Added contains(key) to Metadata

Add contains(key) to org.apache.spark.sql.catalyst.util.Metadata to test the existence of a key. Otherwise, Class Metadata's get methods may throw NoSuchElement exception if the key does not exist.
Testcases are added to MetadataSuite as well.

Author: kai <kaizeng@eecs.berkeley.edu>

Closes #3273 from kai-zeng/metadata-fix and squashes the following commits:

74b3d03 [kai] Added contains(key) to Metadata