Fail test on 8.x because un[sup]]ported is considered valid json key

[andsel]

Build scan:
https://app.travis-ci.com/github/logstash-plugins/logstash-filter-json/jobs/613875477#L1419

Repro line:

export LOGSTASH_PATH=/Users/andrea/workspace/logstash_andsel/ && export LOGSTASH_SOURCE=1
export JRUBY_OPTS="-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED -J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED -J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED -J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED -J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED -J--add-opens=java.base/java.security=ALL-UNNAMED -J--add-opens=java.base/java.io=ALL-UNNAMED -J--add-opens=java.base/java.nio.channels=ALL-UNNAMED -J--add-opens=java.base/sun.nio.ch=ALL-UNNAMED -J--add-opens=java.management/sun.management=ALL-UNNAMED -Xregexp.interruptible=true -Xcompile.invokedynamic=true -Xjit.threshold=0 -J-XX:+PrintCommandLineFlags -v -W1"
bundle install
bundle exec rspec

Reproduces locally?:
yes

Applicable branches:
main

Failure history:

Failure excerpt:

Failures:

  1) LogStash::Filters::Json unsupported field name using `target` "{\"okay\":true, \"un[sup]]ported\": \"foo\", \"another\": \"..." processes events as specified
     Failure/Error: insist { subject.get("tags") }.include?("_jsonparsefailure")

     NoMethodError:
       undefined method `include?' for nil:NilClass
     # /Users/andrea/.rvm/gems/jruby-9.4.2.0/gems/insist-1.0.0/lib/insist/enumerables.rb:11:in `include?'
     # ./spec/filters/json_spec.rb:83:in `block in <main>'
     # /Users/andrea/.rvm/gems/jruby-9.4.2.0/gems/logstash-devutils-2.4.0-java/lib/logstash/devutils/rspec/spec_helper.rb:61:in `block in <main>'

Finished in 2.73 seconds (files took 2.07 seconds to load)
24 examples, 1 failure

Failed examples:

rspec ./spec/filters/json_spec.rb:80 # LogStash::Filters::Json unsupported field name using `target` "{\"okay\":true, \"un[sup]]ported\": \"foo\", \"another\": \"..." processes events as specified

Randomized with seed 39629

Task to do after issue resolution

• Re-enable it in Logstash ci/test_plugins.rb

[andsel]

This is probably fixed by #52