fix: HABTM cant find associations with custom class_name

[adrianthedev]

Fixes #236

[codecov]

Codecov Report

Merging #237 (9fee760) into master (8a13028) will increase coverage by 0.72%.
The diff coverage is 93.60%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   96.03%   96.76%   +0.72%     
==========================================
  Files          87      136      +49     
  Lines        1890     3859    +1969     
==========================================
+ Hits         1815     3734    +1919     
- Misses         75      125      +50     

Impacted Files	Coverage Δ
lib/avo/app/tools_manager.rb	50.00% <50.00%> (ø)
lib/avo/app/fields/belongs_to.rb	90.00% <75.00%> (ø)
lib/avo/app/action.rb	80.48% <80.48%> (ø)
lib/avo/engine.rb	85.71% <85.71%> (ø)
lib/avo/app/fields/boolean_field.rb	93.75% <87.50%> (ø)
lib/avo/app/app.rb	89.61% <89.61%> (ø)
lib/avo/app/resource.rb	90.35% <90.35%> (ø)
lib/avo/app/fields/key_value_field.rb	90.90% <90.90%> (ø)
app/controllers/avo/resources_controller.rb	91.45% <91.45%> (ø)
.../avo/app/fields/field_extensions/has_field_name.rb	91.66% <91.66%> (ø)
... and 176 more

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