Fix TagWidget for Django 2.1

[gasman]

Proposed alternative fix to #505, which preserves the behaviour of the renderer kwarg.

format_value is a more suitable place for converting the value - this way we're sidestepping the need to deal with any current or future changes to Django's render logic.

(Unfortunately the Django <1.10 fallback code makes it pretty ugly, but hopefully that won't be there forever :-) )

[jdufresne]

The project has dropped support for Django <= 1.10. Would you like to rebase this change to account for that?

[gasman]

@jdufresne Sure - rebased now. (Obviously, with #518 merged this change isn't really necessary any more, but I think performing the conversion in format_value is stylistically neater anyhow...)

[codecov-io]

Codecov Report

Merging #534 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   87.81%   87.81%           
=======================================
  Files           5        5           
  Lines         509      509           
  Branches       85       85           
=======================================
  Hits          447      447           
  Misses         49       49           
  Partials       13       13

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[jdufresne]

Thanks!