Prevent defining stubs on undefined methods in LazyDouble

In Crystal 1.6, a segfault would occur in the spec spec/spectator/mocks/lazy_double_spec.cr:238
I suspect this is a Crystal bug of some kind, but can't reduce it.
The methods produced by `method_missing` don't have a return type including Symbol.
Symbol is excluded from the union of return types (Int32 | String | Nil).
The program segfaults when calling a method on the actual value, which is a symbol.
It ultimately crashes when producing a failure message, which indicates the value it tested doesn't equal the expected value (a symbol of the same value).
Avoid this issue by preventing stubs on undefined/untyped methods.

src/spectator/mocks/lazy_double.cr

@@ -26,6 +26,15 @@ module Spectator
       super(_spectator_double_stubs + message_stubs)
     end
 
+    # Defines a stub to change the behavior of a method in this double.
+    #
+    # NOTE: Defining a stub for a method not defined in the double's type raises an error.
+    protected def _spectator_define_stub(stub : Stub) : Nil
+      return super if Messages.types.has_key?(stub.method)
+
+      raise "Can't define stub #{stub} on lazy double because it wasn't initially defined."
+    end
+
     # Returns the double's name formatted for user output.
     private def _spectator_stubbed_name : String
       "#<LazyDouble #{@name || "Anonymous"}>"