Use UserAttrsProxy as last resort in build_initial_user (#259)

[apragacz]

Checklist

• I read Contribution Guidelines
• I ran the checks and the tests (make check && make test) before submitting the PR on my branch and they passed
• I attached unit tests testing the provided code so the code coverage will not drop below 98%

Description

This change modifies build_initial_user which is required for password validation with the following logic:

1. Try to use the provided model with given attributes (previous behavior)
2. If that fails, because e.g. there is custom deserialization for nested elements then use UserAttrsProxy which will just pass the given attributes to validator function via "magic" object.

In most cases of password validation, we will only care about fields like username in that case, so the format of other fields will not affect that logic.

Why the change is needed?

This change is required to solve #259.

Related issues, pull requests, links

• Cannot use a custom registration serializer that has a nested serializer #259

[codecov-commenter]

Codecov Report

Merging #269 (af941ac) into master (bd2fa9c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #269   +/-   ##
=======================================
  Coverage   98.14%   98.15%           
=======================================
  Files          44       44           
  Lines        1562     1570    +8     
  Branches      155      155           
=======================================
+ Hits         1533     1541    +8     
  Misses         16       16           
  Partials       13       13           

Files Changed	Coverage Δ
rest_registration/utils/users.py	95.93% <100.00%> (+0.17%)	⬆️

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