[Spark] do not hardcode the default conf value of READ_SIDE_CHAR_PADD…

…ING (#2861)

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#### Which Delta project/connector is this regarding?
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- [x] Spark
- [ ] Standalone
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## Description

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In `WriteIntoDelta.scala`, there is a small mistake that we hardcode the
default value of `SQLConf.READ_SIDE_CHAR_PADDING` to be `false`, instead
of respecting the config definition.

## How was this patch tested?

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existing test.
## Does this PR introduce _any_ user-facing changes?

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no

spark/src/main/scala/org/apache/spark/sql/delta/commands/WriteIntoDelta.scala

@@ -143,7 +143,7 @@ case class WriteIntoDelta(
         clusterBySpecOpt
       }
     val rearrangeOnly = options.rearrangeOnly
-    val charPadding = sparkSession.conf.get(SQLConf.READ_SIDE_CHAR_PADDING.key, "false") == "true"
+    val charPadding = sparkSession.conf.get(SQLConf.READ_SIDE_CHAR_PADDING)
     val charAsVarchar = sparkSession.conf.get(SQLConf.CHAR_AS_VARCHAR)
     val dataSchema = if (!charAsVarchar && charPadding) {
       data.schema