Feature/xpath transformer (#398)

* XPath transformer, added handlerbars helper to select nodes using xpath and setting the outerxml value in response

* Added test to select attribute value and node text

* Removed extra empty lines

src/WireMock.Net/Transformers/HandleBarsHelpers.cs

@@ -17,6 +17,8 @@ namespace WireMock.Transformers
 
             HandleBarsXeger.Register(handlebarsContext);
 
+            HandleBarsXPath.Register(handlebarsContext);
+
             HandleBarsFile.Register(handlebarsContext, fileSystemHandler);
         }
     }

src/WireMock.Net/Transformers/HandleBarsXPath.cs

@@ -0,0 +1,102 @@
+using HandlebarsDotNet;
+using System;
+using System.Text;
+using System.Xml;
+using System.Xml.XPath;
+using WireMock.Validation;
+#if !NETSTANDARD1_3
+using Wmhelp.XPath2;
+#endif
+
+namespace WireMock.Transformers
+{
+    internal static class HandleBarsXPath
+    {
+        public static void Register(IHandlebars handlebarsContext)
+        {
+            handlebarsContext.RegisterHelper("XPath.SelectSingleNode", (writer, context, arguments) =>
+            {
+                (XPathNavigator nav, string xpath) = ParseArguments(arguments);
+
+                try
+                {
+#if NETSTANDARD1_3
+                    var result = nav.SelectSingleNode(xpath);
+#else
+                    var result = nav.XPath2SelectSingleNode(xpath);
+#endif
+                    writer.WriteSafeString(result.OuterXml);
+                }
+                catch (Exception)
+                {
+                    // Ignore Exception
+                }
+            });
+
+            handlebarsContext.RegisterHelper("XPath.SelectNodes", (writer, context, arguments) =>
+            {
+                (XPathNavigator nav, string xpath) = ParseArguments(arguments);
+
+                try
+                {
+#if NETSTANDARD1_3
+                    var result = nav.Select(xpath);
+#else
+                    var result = nav.XPath2SelectNodes(xpath);
+#endif
+                    var resultXml = new StringBuilder();
+                    foreach (XPathNavigator node in result)
+                    {
+                        resultXml.Append(node.OuterXml);
+                    }
+
+                    writer.WriteSafeString(resultXml);
+                }
+                catch (Exception)
+                {
+                    // Ignore Exception
+                }
+            });
+
+            handlebarsContext.RegisterHelper("XPath.Evaluate", (writer, context, arguments) =>
+            {
+                (XPathNavigator nav, string xpath) = ParseArguments(arguments);
+
+                try
+                {
+#if NETSTANDARD1_3
+                    var result = nav.Evaluate(xpath);
+#else
+                    var result = nav.XPath2Evaluate(xpath);
+#endif
+                    writer.WriteSafeString(result);
+                }
+                catch (Exception)
+                {
+                    // Ignore Exception
+                }
+            });
+        }
+
+        private static (XPathNavigator nav, string xpath) ParseArguments(object[] arguments)
+        {
+            Check.Condition(arguments, args => args.Length == 2, nameof(arguments));
+            Check.NotNull(arguments[0], "arguments[0]");
+            Check.NotNullOrEmpty(arguments[1] as string, "arguments[1]");
+
+            XPathNavigator nav;
+
+            switch (arguments[0])
+            {
+                case string stringValue:
+                    nav = new XmlDocument { InnerXml = stringValue }.CreateNavigator();
+                    break;
+
+                default:
+                    throw new NotSupportedException($"The value '{arguments[0]}' with type '{arguments[0]?.GetType()}' cannot be used in Handlebars XPath.");
+            }
+
+            return (nav, (string)arguments[1]);
+        }
+    }
+}