Fix frozen input for Redis

[TonyCTHsu]

#2369

What does this PR do?

FrozenError, when user provide a frozen hash to Redis.

Instead of mutating the input hash, use Hash#merge to return a new hash.

[ivoanjo]

👍 LGTM

[codecov-commenter]

Codecov Report

Merging #2370 (235abe9) into master (4114496) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2370      +/-   ##
==========================================
- Coverage   98.34%   98.33%   -0.02%     
==========================================
  Files        1101     1101              
  Lines       58940    58943       +3     
==========================================
- Hits        57967    57960       -7     
- Misses        973      983      +10     

Impacted Files	Coverage Δ
lib/datadog/tracing/contrib/redis/patcher.rb	100.00% <100.00%> (ø)
...adog/tracing/contrib/redis/instrumentation_spec.rb	100.00% <100.00%> (ø)
...dog/tracing/contrib/redis/integration_test_spec.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/contrib/redis/miniapp_spec.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/contrib/redis/redis_spec.rb	99.51% <100.00%> (+<0.01%)	⬆️
...ontrib/sidekiq/server_internal_tracer/heartbeat.rb	36.00% <0.00%> (-28.00%)	⬇️
spec/support/synchronization_helpers.rb	81.57% <0.00%> (-2.64%)	⬇️
lib/datadog/core/diagnostics/environment_logger.rb	98.42% <0.00%> (-1.58%)	⬇️

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