[SPARK-8710] [SQL] Change ScalaReflection.mirror from a val to a def.

jira: https://issues.apache.org/jira/browse/SPARK-8710

Author: Yin Huai <yhuai@databricks.com>

Closes #7094 from yhuai/SPARK-8710 and squashes the following commits:

c854baa [Yin Huai] Change ScalaReflection.mirror from a val to a def.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -28,7 +28,11 @@ import org.apache.spark.sql.types._
  */
 object ScalaReflection extends ScalaReflection {
   val universe: scala.reflect.runtime.universe.type = scala.reflect.runtime.universe
-  val mirror: universe.Mirror = universe.runtimeMirror(Thread.currentThread().getContextClassLoader)
+  // Since we are creating a runtime mirror usign the class loader of current thread,
+  // we need to use def at here. So, every time we call mirror, it is using the
+  // class loader of the current thread.
+  override def mirror: universe.Mirror =
+    universe.runtimeMirror(Thread.currentThread().getContextClassLoader)
 }
 
 /**
@@ -39,7 +43,7 @@ trait ScalaReflection {
   val universe: scala.reflect.api.Universe
 
   /** The mirror used to access types in the universe */
-  val mirror: universe.Mirror
+  def mirror: universe.Mirror
 
   import universe._