Workaround for stubbing NoReturn method with Nil

Fixes spec/issues/github_issue_29_spec.cr:15

src/spectator/mocks/stubs.cr

@@ -83,7 +83,14 @@ module Spectator::Mocks
           %harness.mocks.record_call(self, %call)
           if (%stub = %harness.mocks.find_stub(self, %call))
             if typeof({{original}}) == NoReturn
-              return %stub.call!(%args) { raise "Cannot call original implementation of {{name}} - it won't return." }
+              # NOTE: This may break at some point if the Crystal compiler gets smarter.
+              # The `nil` after raise changes the return type of the method from `NoReturn` to `Nil`.
+              # Technically, the `nil` will never be reached and if the compiler realizes it,
+              # the return type could become `NoReturn`
+              return %stub.call!(%args) do
+                raise "Cannot call original implementation of {{name}} - it won't return."
+                nil
+              end
             else
               return %stub.call!(%args) { {{original}} }
             end
@@ -106,7 +113,14 @@ module Spectator::Mocks
           %harness.mocks.record_call(self, %call)
           if (%stub = %harness.mocks.find_stub(self, %call))
             if typeof({{original}}) == NoReturn
-              return %stub.call!(%args) { raise "Cannot call original implementation of {{name}} - it won't return." }
+              # NOTE: This may break at some point if the Crystal compiler gets smarter.
+              # The `nil` after raise changes the return type of the method from `NoReturn` to `Nil`.
+              # Technically, the `nil` will never be reached and if the compiler realizes it,
+              # the return type could become `NoReturn`
+              return %stub.call!(%args) do
+                raise "Cannot call original implementation of {{name}} - it won't return."
+                nil
+              end
             else
               return %stub.call!(%args) { {{original}} { |*%ya| yield *%ya } }
             end