Add config.ToString(bool useFallbackValues) overload to include fallback values (#162)

* Added verification red test

* Added Config.ToString() overload to include fallback values

Author: IgorFedchenko

src/Hocon.Configuration.Test/ConfigurationSpec.cs

@@ -260,6 +260,22 @@ public void CanUseFluentMultiLevelFallback()
             Assert.Equal(-1, config.GetInt("foo.bar.borkbork"));
         }
 
+        [Fact]
+        public void ShouldSerializeFallbackValues()
+        {
+            var a = ConfigurationFactory.ParseString(@" akka : {
+                some-key : value
+            }");
+            var b = ConfigurationFactory.ParseString(@"akka : {
+                other-key : 42
+            }");
+
+            var c = a.WithFallback(b);
+            c.GetInt("akka.other-key").Should().Be(42, "Fallback value should exist as data");
+            c.ToString().Should().NotContain("other-key", "Fallback values are ignored by default");
+            c.ToString(useFallbackValues: true).Should().Contain("other-key", "Fallback values should be displayed when requested");
+        }
+
         [Fact]
         public void CanParseQuotedKeys()
         {

src/Hocon.Configuration/Config.cs

@@ -51,6 +51,22 @@ public Config(HoconRoot source, Config fallback):base(source?.Value, source?.Sub
             Fallback = fallback;
         }
 
+        /// <summary>
+        /// Returns string representation of <see cref="Config"/>, allowing to include fallback values
+        /// </summary>
+        /// <param name="useFallbackValues">If set to <c>true</c>, fallback values are included in the output</param>
+        public string ToString(bool useFallbackValues)
+        {
+            if (!useFallbackValues)
+                return base.ToString();
+
+            var config = this;
+            while (config.Fallback != null)
+                config = config.Fallback;
+
+            return config.ToString();
+        }
+
         /// <summary>
         /// Generates a deep clone of the current configuration.
         /// </summary>