Update multipart.py make_payloads

[SpirosStyliaras]

Fix assertion error when invoking make_payloads with float instance.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 69.41%. Comparing base (bfe63cc) to head (ca652d6).

Files	Patch %	Lines
maas/client/utils/multipart.py	0.00%	1 Missing and 1 partial :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   69.43%   69.41%   -0.03%     
==========================================
  Files          76       76              
  Lines        6024     6026       +2     
  Branches     1347     1020     -327     
==========================================
  Hits         4183     4183              
- Misses       1612     1613       +1     
- Partials      229      230       +1     

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