Fix CollectionDecorator#to_s

Previously it could raise an error when set to infer each item's
decorator.

lib/draper/collection_decorator.rb

@@ -55,7 +55,13 @@ def ==(other)
     end
 
     def to_s
-      "#<CollectionDecorator of #{decorator_class} for #{source.inspect}>"
+      klass = begin
+        decorator_class
+      rescue Draper::UninferrableDecoratorError
+        "inferred decorators"
+      end
+
+      "#<CollectionDecorator of #{klass} for #{source.inspect}>"
     end
 
     def context=(value)

spec/draper/collection_decorator_spec.rb

@@ -286,4 +286,25 @@ def page_number
     end
   end
 
+  describe "#to_s" do
+    subject { Draper::CollectionDecorator.new(source, options) }
+    let(:source) { ["a", "b", "c"] }
+
+    context "when :with option was given" do
+      let(:options) { {with: ProductDecorator} }
+
+      it "returns a string representation of the CollectionDecorator" do
+        subject.to_s.should == '#<CollectionDecorator of ProductDecorator for ["a", "b", "c"]>'
+      end
+    end
+
+    context "when :with option was not given" do
+      let(:options) { {} }
+
+      it "returns a string representation of the CollectionDecorator" do
+        subject.to_s.should == '#<CollectionDecorator of inferred decorators for ["a", "b", "c"]>'
+      end
+    end
+  end
+
 end