Handle case with typeless block

Fixes syntax:
`stub method(&block)`

To stub a block with args, use:
`stub method(&block : Type -> Type)`

Addresses https://github.com/icy-arctic-fox/spectator/issues/42

spec/issues/github_issue_42_spec.cr

@@ -0,0 +1,43 @@
+require "../spec_helper"
+
+abstract class SdkInterface
+  abstract def register_hook(name, &block)
+end
+
+class Example
+  def initialize(@sdk : Sdk)
+  end
+
+  def configure
+    @sdk.register_hook("name") do
+      nil
+    end
+  end
+end
+
+class Sdk < SdkInterface
+  def initialize
+  end
+
+  def register_hook(name, &block)
+    nil
+  end
+end
+
+Spectator.describe Example do
+  mock Sdk do
+    stub register_hook(name, &block)
+  end
+
+  describe "#configure" do
+    it "registers a block on configure" do
+      sdk = Sdk.new
+      example_class = Example.new(sdk)
+      allow(sdk).to receive(register_hook())
+
+      example_class.configure
+
+      expect(sdk).to have_received(register_hook()).with("name")
+    end
+  end
+end