Include stub source in to_s

src/spectator/mocks/generic_method_stub.cr

@@ -20,6 +20,10 @@ module Spectator::Mocks
         io << @args
         io << ')'
       end
+      io << " : "
+      io << ReturnType
+      io << " at "
+      io << @source
     end
   end
 end

src/spectator/mocks/nil_method_stub.cr

@@ -8,7 +8,7 @@ module Spectator::Mocks
       if (cast = nil.as?(RT))
         cast
       else
-        raise "The return type of stub #{to_s} : Nil doesn't match the expected type #{RT}"
+        raise "The return type of stub #{self} doesn't match the expected type #{RT}"
       end
     end
 

src/spectator/mocks/proc_method_stub.cr

@@ -12,7 +12,7 @@ module Spectator::Mocks
       if (cast = result.as?(RT))
         cast
       else
-        raise "The return type of stub #{to_s} : #{ReturnType} doesn't match the expected type #{RT}"
+        raise "The return type of stub #{self} doesn't match the expected type #{RT}"
       end
     end
   end

src/spectator/mocks/value_method_stub.cr

@@ -12,7 +12,7 @@ module Spectator::Mocks
       if (cast = result.as?(RT))
         cast
       else
-        raise "The return type of stub #{to_s} : #{ReturnType} doesn't match the expected type #{RT}"
+        raise "The return type of stub #{self} doesn't match the expected type #{RT}"
       end
     end
   end