Use responds_to? to ensure stub is cast correctly

src/spectator/mocks/lazy_double.cr

@@ -57,9 +57,13 @@ module Spectator
           _spectator_cast_stub_value(%stub, %call, \{{Messages[{{call.name.symbolize}}.id]}}, \{{Messages[{{call.name.symbolize}}.id].resolve >= Nil}})
         \{% else %}
           # A method that was not defined during initialization was stubbed.
+          # Even though all stubs will have a #value method, the compiler doesn't seem to agree.
+          # Assert that it will (this should never fail).
+          raise TypeCastError.new("Stub has no value") unless %stub.responds_to?(:value)
+
           # Return the value of the stub as-is.
           # Might want to give a warning here, as this may produce a "bloated" union of all known stub types.
-          %stub.as(::Spectator::ValueStub).value
+          %stub.value
         \{% end %}
       else
         # A stub wasn't found, invoke the fallback logic.

src/spectator/mocks/null_double.cr

@@ -60,9 +60,13 @@ module Spectator
       # Attempt to find a stub that satisfies the method call and arguments.
       if %stub = _spectator_find_stub(%call)
         # A method that was not defined during initialization was stubbed.
+        # Even though all stubs will have a #value method, the compiler doesn't seem to agree.
+        # Assert that it will (this should never fail).
+        raise TypeCastError.new("Stub has no value") unless %stub.responds_to?(:value)
+
         # Return the value of the stub as-is.
         # Might want to give a warning here, as this may produce a "bloated" union of all known stub types.
-        %stub.as(::Spectator::ValueStub).value
+        %stub.value
       else
         # A stub wasn't found, invoke the fallback logic.
         # Message received for a methods that isn't stubbed nor defined when initialized.

src/spectator/mocks/stubbable.cr

@@ -276,9 +276,14 @@ module Spectator
         %typed.value
       else
         # The stub couldn't be easily cast to match the return type.
+
+        # Even though all stubs will have a #value method, the compiler doesn't seem to agree.
+        # Assert that it will (this should never fail).
+        raise TypeCastError.new("Stub has no value") unless {{stub}}.responds_to?(:value)
+
         # Get the value as-is from the stub.
         # This will be compiled as a union of all known stubbed value types.
-        %value = {{stub}}.as(::Spectator::ValueStub).value
+        %value = {{stub}}.value
 
         # Attempt to cast the value to the method's return type.
         # If successful, which it will be in most cases, return it.