Update rubocop config for proper linting

[inkstak]

FIx rubocop-rspec#1290.
Following discussion in rubocop-rspec#1291.

This PR aims to provide a better rubocop configuration to handle implicit subjects, but also add the following drawback:

its_call(123)         { is_expected.to send_message(Foo, :call).once }
its_call("203000592") { is_expected.to send_message(Foo, :call).once }

RSpec/RepeatedExample: Don't repeat examples within an example group. (https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/RepeatedExample)
  its_call(123)         { is_expected.to send_message(Foo, :call).once }
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RSpec/RepeatedExample: Don't repeat examples within an example group. (https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/RepeatedExample)
  its_call("203000592") { is_expected.to send_message(Foo, :call).once }

These tests are detected as duplicates because of their similar block content.
Their distinct arguments are interpreted as a description.

This can be circumvented by enhancing the tests but that may not be suitable for every scenario:

its_call(123)         { is_expected.to send_message(Foo, :call).once.with(123) }
its_call("203000592") { is_expected.to send_message(Foo, :call).once.with("203000592") }