added test and bugfix for merging querystring parameters

Octokit.Tests/Helpers/UriExtensionsTests.cs

@@ -62,6 +62,21 @@ namespace Octokit.Tests.Helpers
                 Assert.Equal(uri, uriWithNullParameters);
             }
 
+            [Fact]
+            public void CombinesExistingParametersWithNewParameters()
+            {
+                var uri = new Uri("https://api.github.com/repositories/1/milestones?state=closed&sort=due_date&direction=asc&page=2");
+
+                var parameters = new Dictionary<string, string> { { "state", "open" }, { "sort", "other"} };
+
+                var actual = uri.ApplyParameters(parameters);
+
+                Assert.True(actual.Query.Contains("state=open"));
+                Assert.True(actual.Query.Contains("sort=other"));
+                Assert.True(actual.Query.Contains("direction=asc"));
+                Assert.True(actual.Query.Contains("page=2"));
+            }
+
             [Fact]
             public void EnsuresArgumentNotNull()
             {

Octokit/Helpers/UriExtensions.cs

@@ -12,6 +12,19 @@ namespace Octokit
 
             if (parameters == null || !parameters.Any()) return uri;
 
+            var existingParameters = uri.Query.Split(new[] { '&' })
+                .ToDictionary(
+                    key => key.Substring(0, key.IndexOf('=')),
+                    value => value.Substring(value.IndexOf('=') + 1));
+
+            foreach (var existing in existingParameters)
+            {
+                if (!parameters.ContainsKey(existing.Key))
+                {
+                    parameters.Add(existing);
+                }
+            }
+
             string query = String.Join("&", parameters.Select(kvp => kvp.Key + "=" + kvp.Value));
             if (uri.IsAbsoluteUri)
             {