Minor cleanup to Milestone model and spec

Back-ported from my still-in-progress major cleanup.

spec/models/milestone_spec.rb

@@ -31,24 +31,33 @@ describe Milestone do
 
   it { milestone.should be_valid }
 
-  describe "Issues" do 
-    before do 
+  describe "#percent_complete" do
+    it "should not count open issues" do
       milestone.issues << issue
+      milestone.percent_complete.should == 0
     end
 
-    it { milestone.percent_complete.should == 0 }
+    it "should count closed issues" do
+      issue.update_attributes(closed: true)
+      milestone.issues << issue
+      milestone.percent_complete.should == 100
+    end
 
-    it do 
-      issue.update_attributes closed: true
+    it "should recover from dividing by zero" do
+      milestone.issues.should_receive(:count).and_return(0)
       milestone.percent_complete.should == 100
     end
   end
 
-  describe :expires_at do 
-    before do 
-      milestone.update_attributes due_date: Date.today + 1.day
+  describe "#expires_at" do
+    it "should be nil when due_date is unset" do
+      milestone.update_attributes(due_date: nil)
+      milestone.expires_at.should be_nil
     end
 
-    it { milestone.expires_at.should_not be_nil }
+    it "should not be nil when due_date is set" do
+      milestone.update_attributes(due_date: Date.tomorrow)
+      milestone.expires_at.should be_present
+    end
   end
 end