Enable support for milestone-based issues queries (#1788)

* Added support for Milestone filter in SearchIssuesRequest

* Fixed some styling issues to match with the style of the existing code

* Wrap milestone value with double quotes as milestones can contain spaces

* Allow milestone filter to contain double quotes

* Ability to search by milestone exclusions

* Moved the EscapeDoubleQuotes method to StringExtensions

Octokit.Tests/Helpers/StringExtensionsTests.cs

@@ -64,5 +64,30 @@ namespace Octokit.Tests.Helpers
                 Assert.Equal(expected, template.ExpandUriTemplate(new { name = "example name.txt", label = "labeltext" }).ToString());
             }
         }
+
+        public class EscapeDoubleQuotesMethod
+        {
+            [Fact]
+            public void EscapeDoubleQuotesReturnsNullForNullInput()
+            {
+                Assert.Equal(null, (null as string).EscapeDoubleQuotes());
+            }
+
+            [Fact]
+            public void EscapeDoubleQuotesReturnsInputWithoutDoubleQuotes()
+            {
+                string input = "some test input without double quotes in it";
+
+                Assert.Equal(input, input.EscapeDoubleQuotes());
+            }
+
+            [Fact]
+            public void EscapeDoubleQuotesEscapesAllDoubleQuotes()
+            {
+                string input = "\"test milestone\"";
+
+                Assert.Equal("\\\"test milestone\\\"", input.EscapeDoubleQuotes());
+            }
+        }
     }
 }