Support decimal type properly

Redshift handles Decimal as string currently when loading data from Avro format since Redshift used older version of Avro
The Avro supported Decimal type natively from 1.7.7 - https://issues.apache.org/jira/browse/AVRO-1402

src/main/scala/com/databricks/spark/redshift/RedshiftWriter.scala

@@ -223,6 +223,7 @@ private[redshift] class RedshiftWriter(
     // However, each task gets its own deserialized copy, making this safe.
     val conversionFunctions: Array[Any => Any] = data.schema.fields.map { field =>
       field.dataType match {
+        case _: DecimalType => (v: Any) => if (v == null) null else v.toString
         case DateType =>
           val dateFormat = Conversions.createRedshiftDateFormat()
           (v: Any) => {
@@ -271,6 +272,8 @@ private[redshift] class RedshiftWriter(
     // strings. This is necessary for Redshift to be able to load these columns (see #39).
     val convertedSchema: StructType = StructType(
       schemaWithLowercaseColumnNames.map {
+        case StructField(name, _: DecimalType, nullable, meta) =>
+          StructField(name, StringType, nullable, meta)
         case StructField(name, DateType, nullable, meta) =>
           StructField(name, StringType, nullable, meta)
         case StructField(name, TimestampType, nullable, meta) =>