Disallow stubs for undefined methods in null double

Any method_missing invocation should return self.

spec/spectator/mocks/null_double_spec.cr

@@ -216,12 +216,12 @@ Spectator.describe Spectator::NullDouble do
         expect(dbl.foo("foobar")).to eq("bar")
       end
 
-      it "raises self when constraint unsatisfied" do
+      it "raises when constraint unsatisfied" do
         expect { dbl.foo("baz") }.to raise_error(Spectator::UnexpectedMessage, /foo/)
       end
 
-      it "raises self when argument count doesn't match" do
-        expect { dbl.foo }.to raise_error(Spectator::UnexpectedMessage, /foo/)
+      it "returns self when argument count doesn't match" do
+        expect(dbl.foo).to be(dbl)
       end
 
       it "ignores the block argument if not in the constraint" do
@@ -406,15 +406,6 @@ Spectator.describe Spectator::NullDouble do
     it "removes previously defined stubs" do
       expect { dbl._spectator_clear_stubs }.to change { dbl.foo }.from(5).to(42)
     end
-
-    it "defaults to returning itself for methods with no implementation" do
-      stub = Spectator::ValueStub.new(:baz, :xyz)
-      dbl._spectator_define_stub(stub)
-      expect(dbl.baz).to eq(:xyz)
-
-      dbl._spectator_clear_stubs
-      expect(dbl.baz).to be(dbl)
-    end
   end
 
   describe "#_spectator_calls" do

src/spectator/mocks/null_double.cr

@@ -58,21 +58,7 @@ module Spectator
       %call = ::Spectator::MethodCall.new({{call.name.symbolize}}, %args)
       _spectator_record_call(%call)
 
-      # 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 #call 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?(:call)
-
-        # 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.call(%call)
-      else
-        # A stub wasn't found, invoke the fallback logic.
-        # Message received for a methods that isn't stubbed nor defined when initialized.
-        _spectator_abstract_stub_fallback(%call)
-      end
+      self
     end
   end
 end