jio_ui_test: explicitly set Content-Type header when returning HTML

This is no longer guessed on Zope4
12 jobs for zope4py2 in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Zope4py2

02:03:38

failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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canceled SlapOS-Master.UnitTest-Zope4py2

failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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failed SlapOS-Master.UnitTest-Zope4py2

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