Do not report if Object#to_s is called without receiver

spec/ameba/rule/lint/redundant_string_cercion_spec.cr

@@ -52,6 +52,20 @@ module Ameba::Rule::Lint
       subject.catch(s).should_not be_valid
     end
 
+    it "doesn't report if Object#to_s is called with arguments" do
+      s = Source.new %q(
+        /\w #{name.to_s(io)}/
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it "doesn't report if Object#to_s is called without receiver" do
+      s = Source.new %q(
+        /\w #{to_s}/
+      )
+      subject.catch(s).should be_valid
+    end
+
     it "reports rule, location and message" do
       s = Source.new %q(
         "Hello, #{name1.to_s}"

src/ameba/rule/lint/redundant_string_coercion.cr

@@ -36,7 +36,7 @@ module Ameba::Rule::Lint
 
     private def string_coercion_nodes(node)
       node.expressions.select do |e|
-        e.is_a?(Crystal::Call) && e.name == "to_s" && e.args.size == 0
+        e.is_a?(Crystal::Call) && e.name == "to_s" && e.args.size.zero? && e.obj
       end
     end
   end