Fix multi-fields in single line

[morsik]

As Django documentation states, you may use:

{
    "fields": [("first_name", "last_name"), "address", "city", "state"],
}

to specify that first_name and last_name should be displayed in single line.

Django Polymorphic though wasn't able to work with this syntax properly as it tried to remove whole tuple instead of individual elements from child fields.

This PR fixes this, and now multi-field lines are rendered correctly.

[morsik]

@jleclanche have you checked this PR? I totally forgot about this, but after reading it right now again, it seems I did big mistake in it!

In this PR I removed for field in fieldset[1]["fields"]: in this PR, but I think it should stayed as it was, and all my code and else statement with original code should be just inside for loop...

Otherwise theres no field available to use with my if isinstance(field, tuple) :/
Sorry!

[jleclanche]

@j-antunes could you submit the corrected version you merged recently?

[morsik]

@jleclanche fixed, see PR above ^

@j-antunes actually copied correct part of the code in his commit from my PR without removing that one line that shouldn't be removed.

[j-antunes]

@morsik - Awesome thank you!