Sets Content-Length to body string length

It joins the body array to a string and sets the Content-Length header
to its length.

lib/zero/response.rb

@@ -42,7 +42,7 @@ module Zero
     # Also creates one, if it does not exists
     #
     def content_length
-      header['Content-Length'] = 0
+      header['Content-Length'] = body.join.length
     end
 
   end

spec/unit/response/response_spec.rb

@@ -53,6 +53,13 @@ describe Zero::Response do
 
       subject.header['Content-Length'].should eq(0)
     end
+
+    it "sets the content_length to the size of the message body" do
+      subject.body = ['foo', 'bar']
+      subject.content_length
+
+      subject.header['Content-Length'].should eq(6)
+    end
   end
 
 end