Fix invalid/non-representative tests

[jdufresne]

Passing bytes objects to a model CharField (a string field) is not
supported and incorrect use of the API. When Python is started with -b,
it can result in the warnings:

.../django/db/models/fields/__init__.py:1078: BytesWarning: str() on a bytes instance
  return str(value)
.../django/db/models/fields/__init__.py:1078: BytesWarning: str() on a bytes instance
  return str(value)

Avoid this warning by passing str values, as expected.

For details on the Python -b flag, see:

https://docs.python.org/3/using/cmdline.html#miscellaneous-options

[codecov]

Codecov Report

Merging #1064 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
- Coverage   84.08%   83.93%   -0.16%     
==========================================
  Files          24       24              
  Lines        1301     1301              
  Branches      178      178              
==========================================
- Hits         1094     1092       -2     
- Misses        157      159       +2     
  Partials       50       50

Impacted Files	Coverage Δ
debug_toolbar/panels/sql/panel.py	62.41% <0%> (-1.35%)	⬇️

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[matthiask]

Hmm. It is incorrect use of the queryset API, but doesn't it still make sense to test that django-debug-toolbar doesn't crash in this case?

[jdufresne]

A use case for binary values is querying by a BinaryField. I'll see if there is a better way to handle this.