Add Configuration#key?

lib/nanoc/base/entities/configuration.rb

@@ -51,6 +51,10 @@ def to_h
       @wrapped
     end
 
+    def key?(key)
+      @wrapped.key?(key)
+    end
+
     def [](key)
       @wrapped[key]
     end

spec/nanoc/base/entities/configuration_spec.rb

@@ -0,0 +1,19 @@
+describe Nanoc::Int::Configuration do
+  let(:configuration) { described_class.new(hash) }
+
+  let(:hash) { { foo: 'bar' } }
+
+  describe '#key?' do
+    subject { configuration.key?(key) }
+
+    context 'non-existent key' do
+      let(:key) { :donkey }
+      it { is_expected.not_to be }
+    end
+
+    context 'existent key' do
+      let(:key) { :foo }
+      it { is_expected.to be }
+    end
+  end
+end

spec/nanoc/base/views/config_view_spec.rb

@@ -1,8 +1,10 @@
 describe Nanoc::ConfigView do
   let(:config) do
-    { amount: 9000, animal: 'donkey' }
+    Nanoc::Int::Configuration.new(hash)
   end
 
+  let(:hash) { { amount: 9000, animal: 'donkey' } }
+
   let(:view) { described_class.new(config, nil) }
 
   describe '#frozen?' do

spec/nanoc/regressions/gh_815_spec.rb

@@ -0,0 +1,18 @@
+describe 'GH-813', site: true, stdio: true do
+  before do
+    File.write('nanoc.yaml', "animal: \"donkey\"\n")
+    File.write('content/foo.md', '<%= @config.key?(:animal) %>')
+    File.write('Rules', <<EOS)
+  compile '/**/*' do
+    filter :erb
+    write item.identifier.without_ext + '.txt'
+  end
+EOS
+  end
+
+  specify 'Nanoc generates diff for proper path' do
+    Nanoc::CLI.run(['compile'])
+
+    expect(File.read('output/foo.txt')).to eql('true')
+  end
+end