[SPARK-18055][SQL] Use correct mirror in ExpresionEncoder

Previously, we were using the mirror of passed in `TypeTag` when reflecting to build an encoder.  This fails when the outer class is built in (i.e. `Seq`'s default mirror is based on root classloader) but inner classes (i.e. `A` in `Seq[A]`) are defined in the REPL or a library.

This patch changes us to always reflect based on a mirror created using the context classloader.

Author: Michael Armbrust <michael@databricks.com>

Closes #17201 from marmbrus/replSeqEncoder.

(cherry picked from commit 314e48a3)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>