Migrate ActionView::Utils tests to RSpec

[marcotc]

Same test, different testing framework.

[codecov-io]

Codecov Report

Merging #1432 (73ff8c6) into master (000e167) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1432   +/-   ##
=======================================
  Coverage   98.13%   98.14%           
=======================================
  Files         771      772    +1     
  Lines       36901    36926   +25     
=======================================
+ Hits        36214    36241   +27     
+ Misses        687      685    -2     

Impacted Files	Coverage Δ
spec/ddtrace/contrib/action_view/utils_spec.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/action_view/utils.rb	100.00% <0.00%> (+14.28%)	⬆️

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[ivoanjo]

Thank you 🙇