RedundantStringCoercion: do not report to_s with named args

closes #160

spec/ameba/rule/lint/redundant_string_cercion_spec.cr

@@ -66,6 +66,13 @@ module Ameba::Rule::Lint
       subject.catch(s).should be_valid
     end
 
+    it "doesn't report if Object#to_s is called with named args" do
+      s = Source.new %q(
+        "0x#{250.to_s base: 16}"
+      )
+      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 +36,11 @@ 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.zero? && e.obj
+        e.is_a?(Crystal::Call) &&
+          e.name == "to_s" &&
+          e.args.size.zero? &&
+          e.named_args.nil? &&
+          e.obj
       end
     end
   end