[SPARK-21849][Core]Make the serializer function more robust

[libratiger]

What changes were proposed in this pull request?

make sure the close function is called in the finally block.

How was this patch tested?

No Test, just compile.

[srowen]

There is no actual resource associated with the underlying stream anyway (it's a ByteArrayOutputStream) and it's closed in the normal flow anyway. I don't object, but it's the kind of thing that's generally too trivial to spend time proposing

[libratiger]

Yes, I agree. it is better included in a normal pull request.

[SparkQA]

Test build #3908 has finished for PR 19067 at commit b523ecb.

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

[srowen]

If this doesn't fix anything and isn't making the code better or more consistent, it should be closed