New rule: useless condition in when

thanks to @hugoabonizio for suggesstion

spec/ameba/rule/useless_condition_in_when_spec.cr

@@ -0,0 +1,45 @@
+require "../../spec_helper"
+
+module Ameba::Rule
+  subject = UselessConditionInWhen.new
+
+  describe UselessConditionInWhen do
+    it "passes if there is not useless condition" do
+      s = Source.new %(
+        case
+        when utc?
+          io << " UTC"
+        when local?
+          Format.new(" %:z").format(self, io) if utc?
+        end
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it "fails if there is useless if condition" do
+      s = Source.new %(
+        case
+        when utc?
+          io << " UTC" if utc?
+        end
+      )
+      subject.catch(s).should_not be_valid
+    end
+
+    it "reports rule, location and message" do
+      s = Source.new %(
+        case
+        when String
+          puts "hello"
+        when can_generate?
+          generate if can_generate?
+        end
+      ), "source.cr"
+      subject.catch(s).should_not be_valid
+      error = s.errors.first
+      error.rule.should_not be_nil
+      error.location.to_s.should eq "source.cr:6:23"
+      error.message.should eq "Useless condition in when detected"
+    end
+  end
+end