Fix ActionPack instrumentation #starts_with? error

[delner]

When running ActionPack standalone (without Rails), ActionController instrumentation may raise undefined method 'starts_with?' when processing requests. This bug was discovered when running the Rails test suite with Datadog tracing enabled.

This occurs because starts_with? is an ActiveSupport function, which may not be available. This pull request changes the function to start_with? which is the Ruby core library version, so it should be available outside Rails as well.

This bug was not caught earlier because we have a gap in our test coverage over this controller instrumentation. This pull request tests for this specific bug, but does not fully address that gap: something we will want to do soon, possibly accompanying a refactor of the instrumentation itself.

[codecov-commenter]

Codecov Report

Merging #1489 (8a35b6e) into master (6ecfa4e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1489      +/-   ##
==========================================
- Coverage   98.22%   98.22%   -0.01%     
==========================================
  Files         860      861       +1     
  Lines       41323    41361      +38     
==========================================
+ Hits        40589    40626      +37     
- Misses        734      735       +1     

Impacted Files	Coverage Δ
...b/action_pack/action_controller/instrumentation.rb	94.28% <100.00%> (-1.43%)	⬇️
lib/ddtrace/contrib/action_pack/utils.rb	100.00% <100.00%> (ø)
...ion_pack/action_controller/instrumentation_spec.rb	100.00% <100.00%> (ø)

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