[SPARK-4791] [sql] Infer schema from case class with multiple constructors

Modified ScalaReflection.schemaFor to take primary constructor of Product when there are multiple constructors.  Added test to suite which failed before but works now.

Needed for [https://github.com/apache/spark/pull/3637]

CC: marmbrus

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #3646 from jkbradley/sql-reflection and squashes the following commits:

796b2e4 [Joseph K. Bradley] Modified ScalaReflection.schemaFor to take primary constructor of Product when there are multiple constructors.  Added test to suite which failed before but works now.