[SPARK-11793][SQL] Dataset should set the resolved encoders internally for maps.

[rxin]

I also wrote a test case -- but unfortunately the test case is not working due to SPARK-11795.

[cloud-fan]

Actually we don't need this assert, encoderFor already did this.

[rxin]

I'm going to merge this and fix the comments in a separate pull request.

[SparkQA]

Test build #46133 has finished for PR 9784 at commit ae96866.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class HashingTF(override val uid: String)\n * class Interaction @Since(\"1.6.0\") (override val uid: String) extends Transformer\n * class Normalizer(override val uid: String)\n * class SQLTransformer @Since(\"1.6.0\") (override val uid: String) extends Transformer with Writable\n * class Tokenizer(override val uid: String)\n * case class ArrayConversion(elementConversion: JDBCConversion) extends JDBCConversion\n * abstract class JdbcDialect extends Serializable\n