Warn when placeholders are redefined

If a placeholder is replaced a warning is now emitted. As the test suite
relied on placeholder replacements being silent, placeholders are now
cleared to defaults after each test case. Internal calls made from dsl.rb are filtered
out in order to show the real location of the replacement.

lib/turnip/placeholder.rb

@@ -4,6 +4,11 @@ class Match < Struct.new(:regexp, :block);end
 
     class << self
       def add(name, &block)
+        if placeholders.key?(name)
+          location = caller_locations.detect { |l| l.to_s !~ /lib\/turnip\/dsl\.rb/ }
+          warn "Placeholder :#{name} was replaced at #{location}."
+        end
+
         placeholders[name] = Placeholder.new(name, &block)
       end
 

spec/placeholder_spec.rb

@@ -9,6 +9,8 @@
       end
     end
 
+    after { Turnip::Placeholder.send(:placeholders).clear }
+
     it 'returns a regexp for the given placeholder' do
       resolved = described_class.resolve(:test)
 
@@ -150,4 +152,19 @@
       end
     end
   end
+
+  describe 'replacing placeholders' do
+    before do
+      described_class.add(:test) do
+        match(/foo/)
+      end
+    end
+
+    it 'issues a warning' do
+      expect(described_class).to receive(:warn).with(/Placeholder :test was replaced/)
+      described_class.add(:test) do
+        match(/bar/)
+      end
+    end
+  end
 end

spec/step_definition_spec.rb

@@ -3,6 +3,8 @@
 describe Turnip::StepDefinition do
   let(:all_steps) { [] }
 
+  after { Turnip::Placeholder.send(:placeholders).clear }
+
   describe "#match" do
     it "matches a simple step" do
       step = Turnip::StepDefinition.new("there are monsters") {}