[SPARK-16297][SQL] Fix mapping Microsoft SQLServer dialect

The problem is if it is run with no fix throws an exception and causes the following error:

  "Cannot specify a column width on data type bit."

The problem stems from the fact that the "java.sql.types.BIT" type is mapped as BIT[n] that really must be mapped as BIT.
This concerns the type Boolean.

As for the type String with maximum length of characters it must be mapped as VARCHAR (MAX) instead of TEXT which is a type deprecated in SQLServer.

Here is the list of mappings for SQL Server:
https://msdn.microsoft.com/en-us/library/ms378878(v=sql.110).aspx

Closes #13944 from meknio/master.

sql/core/src/main/scala/org/apache/spark/sql/jdbc/MsSqlServerDialect.scala

@@ -36,6 +36,8 @@ private object MsSqlServerDialect extends JdbcDialect {
 
   override def getJDBCType(dt: DataType): Option[JdbcType] = dt match {
     case TimestampType => Some(JdbcType("DATETIME", java.sql.Types.TIMESTAMP))
+    case StringType => Some(JdbcType("NVARCHAR(MAX)", java.sql.Types.NVARCHAR))
+    case BooleanType => Some(JdbcType("BIT", java.sql.Types.BIT))
     case _ => None
   }