Added the test cases for content_for?

Signed-off-by: Ilya Shindyapin <ilya@shindyapin.com>

sinatra-contrib/spec/content_for/footer.erb

@@ -0,0 +1,3 @@
+<% if content_for? :foo %>
+  <%= yield_content :foo %>
+<% end %>

sinatra-contrib/spec/content_for/footer.erubis

@@ -0,0 +1,3 @@
+<% if content_for? :foo %>
+  <%= yield_content :foo %>
+<% end %>

sinatra-contrib/spec/content_for/footer.haml

@@ -0,0 +1,2 @@
+- if content_for? :foo
+	= yield_content :foo

sinatra-contrib/spec/content_for/footer.slim

@@ -0,0 +1,2 @@
+- if content_for? :foo
+	= yield_content :foo

sinatra-contrib/spec/content_for_spec.rb

@@ -138,6 +138,18 @@ describe Sinatra::ContentFor do
           render(inner, :passes_values).should == "<i>1</i>2"
         end
       end
+      
+      describe "with content_for? in Ruby" do
+        it 'renders block if key is set' do 
+          content_for(:foo) { "foot" }
+          render(inner, :footer).should == "foot"
+        end
+        
+        it 'does not render a block if different key' do
+          content_for(:different_key) { "foot" }
+          render(inner, :footer).should be_empty
+        end
+      end
 
       engines.each do |outer|
         describe "with yield_content in #{outer.capitalize}" do