fix partials used with blocks

middleman-core/features/partials.feature

@@ -50,3 +50,10 @@ Feature: Provide Sane Defaults for Partial Behavior
     Then I should see "File Not Found"
     When I go to "/_code_snippet.html"
     Then I should see "File Not Found"
+
+Scenario: Works with blocks
+    Given the Server is running at "partials-app"
+    When I go to "/block.html"
+    Then I should see "Start"
+    And I should see "Contents"
+    And I should see "End"

middleman-core/fixtures/partials-app/source/_block.erb

@@ -0,0 +1,3 @@
+Start
+<%= yield %>
+End

middleman-core/fixtures/partials-app/source/block.html.erb

@@ -0,0 +1,3 @@
+<% partial "block" do %>
+Contents 
+<% end %>

middleman-core/lib/middleman-core/template_context.rb

@@ -96,8 +96,9 @@ module Middleman
     #
     # @param [String, Symbol] name The partial to render.
     # @param [Hash] options
+    # @param [Proc] block A block will be evaluated to return internal contents.
     # @return [String]
-    Contract Any, Or[Symbol, String], Hash => String
+    Contract Any, Or[Symbol, String], Hash => String, Maybe[Proc] => String
     def render(_, name, options={}, &block)
       name = name.to_s