Fix a false positive for Lint/NumberConversion

[koic]

This PR fixes the following false positive and incorrect auto-correct for Lint/NumberConversion when :to_f is one of multiple method arguments.

% cat example.rb
delegate :to_f, to: :description, allow_nil: true

% bundle exec rubocop --only Lint/NumberConversion -A example.rb
(snip)

Inspecting 1 file
W

Offenses:

example.rb:1:1: W: [Corrected] Lint/NumberConversion: Replace unsafe
number conversion with number class parsing, instead of using :to_f, use
stricter { |i| Float(i) }.
delegate :to_f, to: :description, allow_nil: true
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

% cat example.rb
delegate { |i| Float(i) }, to: :description, allow_nil: true

% ruby -c example.rb
example.rb:1: syntax error, unexpected ',', expecting end-of-input
delegate { |i| Float(i) }, to: :description, allow_nil:...

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!