fix datatype to match the database

[kimakan]

see #123

[coveralls]

Pull Request Test Coverage Report for Build 3757674568

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0%) to 65.883%

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Totals
Change from base Build 3619221320:	0.0%
Covered Lines:	4598
Relevant Lines:	6979

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💛 - Coveralls

[harenk]

I approve of this change, since it enables to use astropy/pyVO TAP queries on the metadata tables