Don't fail erroneously on unstub_const when not stubbed

It's possible to call `unstub_const` without having called `stub_const`,
or while having already called `unstub_const`. This can happen when
there is an exception while setting up the model itself in the before
block, and `stub_const` is never called, but the after block still tries
to cleanup by calling `unstub_const`.

lib/with_model/constant_stubber.rb

@@ -19,9 +19,11 @@ def stub_const(value)
     end
 
     def unstub_const
-      @namespace.__send__ :remove_const, basename
-      @namespace.const_set basename, @original_value if @original_value
-      @namespace = nil
+      if @namespace
+        @namespace.__send__ :remove_const, basename
+        @namespace.const_set basename, @original_value if @original_value
+        @namespace = nil
+      end
       @original_value = nil
     end
 

spec/constant_stubber_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+module WithModel
+  describe ConstantStubber do
+    it 'allows calling unstub_const multiple times' do
+      stubber = described_class.new('Foo')
+      stubber.stub_const(1)
+      stubber.unstub_const
+      stubber.unstub_const
+    end
+
+    it 'allows calling unstub_const without stub_const' do
+      stubber = described_class.new('Foo')
+      stubber.unstub_const
+    end
+  end
+end