[SPARK-15241][SPARK-15242][SQL] fix 2 decimal-related issues in RowEncoder

[cloud-fan]

What changes were proposed in this pull request?

SPARK-15241: We now support java decimal and catalyst decimal in external row, it makes sense to also support scala decimal.

SPARK-15242: This is a long-standing bug, and is exposed after #12364, which eliminate the If expression if the field is not nullable:

val fieldValue = serializerFor(
  GetExternalRowField(inputObject, i, externalDataTypeForInput(f.dataType)),
  f.dataType)
if (f.nullable) {
  If(
    Invoke(inputObject, "isNullAt", BooleanType, Literal(i) :: Nil),
    Literal.create(null, f.dataType),
    fieldValue)
} else {
  fieldValue
}

Previously, we always use DecimalType.SYSTEM_DEFAULT as the output type of converted decimal field, which is wrong as it doesn't match the real decimal type. However, it works well because we always put converted field into If expression to do the null check, and If use its trueValue's data type as its output type.
Now if we have a not nullable decimal field, then the converted field's output type will be DecimalType.SYSTEM_DEFAULT, and we will write wrong data into unsafe row.

The fix is simple, just use the given decimal type as the output type of converted decimal field.

These 2 issues was found at #13008

How was this patch tested?

new tests in RowEncoderSuite

[cloud-fan]

cc @yhuai @liancheng @viirya

[SparkQA]

Test build #58202 has finished for PR 13019 at commit 9621fc6.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

actually cc @davies also

[yhuai]

I feel it is still good to keep int, string, and double columns?

[viirya]

LGTM

[SparkQA]

Test build #58339 has finished for PR 13019 at commit a2c77ca.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[davies]

LGTM

[davies]

Merging this into master and 2.0, thanks!