Tested out all of the existing Pull Request code

Still missing integration tests for Pull Requests, though.  I need to do
a bit more research before I can start to tackle that one.

Octokit.Tests/Models/PullRequestRequestTests.cs

@@ -0,0 +1,35 @@
+using Octokit;
+using Xunit;
+
+public class PullRequestRequestTests
+{
+    public class TheToParametersDictionaryMethod
+    {
+        [Fact]
+        public void ContainsSetValues()
+        {
+            var request = new PullRequestRequest
+            {
+                State = ItemState.Closed,
+                Head = "user:ref-name",
+                Base = "fake_base_branch"
+            };
+
+            var parameters = request.ToParametersDictionary();
+
+            Assert.Equal("closed", parameters["state"]);
+            Assert.Equal("user:ref-name", parameters["head"]);
+            Assert.Equal("fake_base_branch", parameters["base"]);
+        }
+
+        [Fact]
+        public void ReturnsDefaultValuesForDefaultRequest()
+        {
+            var request = new PullRequestRequest();
+
+            var parameters = request.ToParametersDictionary();
+
+            Assert.Equal("open", parameters["state"]);
+        }
+    }
+}