support ArrayType(StringType, _) for SequenceExample

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[zhangxuhong]

i signed

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[jhseu]

@skavulya Would you mind taking a look if you have time?

@zhangxuhong Mind adding a test?

[zhangxuhong]

@jhseu Thanks for taking a look at it. If this issue can be confirmed, i will add the corresponding tests.

[skavulya]

@zhangxuhong Thanks for the contribution. I'll take a look at it this evening to confirm the issue.

[skavulya]

@zhangxuhong Yes, this is an issue. Please add the unit tests. Thanks.