Fix a false positive for Rails/DynamicFindBy

This PR fixes a false positive for `Rails/DynamicFindBy`
when any of the arguments are splat argument.

```console
% cat example.rb
find_by_scan(*args)

% bundle exec rubocop -a example.rb
(snip)

Inspecting 1 file
E

Offenses:

example.rb:1:1: C: [Corrected] Rails/DynamicFindBy: Use find_by instead
of dynamic find_by_scan.
find_by_scan(*args)
^^^^^^^^^^^^^^^^^^^
example.rb:1:15: E: Lint/Syntax: unexpected token tSTAR
(Using Ruby 2.7 parser; configure using TargetRubyVersion parameter,
under AllCops)
find_by(scan: *args)
              ^

1 file inspected, 2 offenses detected, 1 offense corrected

% cat example.rb
find_by(scan: *args)

% ruby -c example.rb
example.rb:1: syntax error, unexpected *
find_by(scan: *args)
```

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Bug fixes
 
 * [#345](https://github.com/rubocop-hq/rubocop-rails/issues/345): Fix error of `Rails/AfterCommitOverride` on `after_commit` with a lambda. ([@pocke][])
+* [#346](https://github.com/rubocop-hq/rubocop-rails/pull/346): Fix a false positive for `Rails/DynamicFindBy` when any of the arguments are splat argument. ([@koic][])
 
 ## 2.8.0 (2020-09-04)
 

lib/rubocop/cop/rails/dynamic_find_by.rb

@@ -41,6 +41,7 @@ def on_send(node)
           method_name = node.method_name
           static_name = static_method_name(method_name)
           return unless static_name
+          return if node.arguments.any?(&:splat_type?)
 
           add_offense(node,
                       message: format(MSG, static_name: static_name,

spec/rubocop/cop/rails/dynamic_find_by_spec.rb

@@ -133,6 +133,14 @@
     expect_no_offenses('User.find_by(name: name)')
   end
 
+  it 'accepts splat argument' do
+    expect_no_offenses('User.find_by_scan(*args)')
+  end
+
+  it 'accepts any of the arguments are splat argument' do
+    expect_no_offenses('User.find_by_foo_and_bar(arg, *args)')
+  end
+
   it 'accepts method in whitelist' do
     expect_no_offenses(<<~RUBY)
       User.find_by_sql(["select * from users where name = ?", name])