Make readonly collections truly readonly

These methods actually returned mutable collections that happen
to implement the readonly interfaces. User could cast them to
the actual type and add things. I'd rather avoid even that
possibility by making these truly return readonly stuff.

Octokit.Tests/Models/SearchCodeRequestTests.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+using Octokit;
+using Xunit;
+
+public class SearchCodeRequestTests
+{
+    public class TheMergedQualifiersMethod
+    {
+        [Fact]
+        public void ReturnsAReadOnlyDictionary()
+        {
+            var request = new SearchCodeRequest("test");
+            
+            var result = request.MergedQualifiers();
+
+            // If I can cast this to a writeable collection, then that defeats the purpose of a read only.
+            Assert.Null(result as ICollection<string>);
+        }
+    }
+}