Use actual value if label is empty

This will show a value instead of nothing if the should-syntax is used.

src/spectator/expectation.cr

@@ -2,9 +2,12 @@ require "./matchers/matcher"
 
 module Spectator
   class Expectation(T)
-    protected getter label : String
     getter actual : T
 
+    protected def label : String
+      @label.empty? ? @actual.to_s : @label
+    end
+
     protected def initialize(@label : String, @actual : T)
     end