Improved relative path checking based on file existence (#411)

* Improved relative path checking based on file existence

If the file exists at the relative path, then use it. If not, then use the path as is.

* Apply File.Exists logic to ReadResponseBodyAsString as well

* Make path handling more robust since path is user defined

* Unit tests for relative path feature

* Replace all back and forward slashes with system dependent DirectorySeparatorChar

* Attempt fix broken directory separator chars for Unix platforms

* Revert wrapping GetMappingFolder with CleanPath

* Move CleanPath logic to its own class

* Remove whitespace

* Remove more whitespace

* Improve CleanPath method

* Move PathUtils tests to separate class

Add another test to ResponseWithBodyFromFileTests

* Fix Response_ProvideResponse_WithBodyFromFile_InAdminMappingFolder

* Debug Linux CI build

* Debug Linux CI

* print all files from admin mappings folder

* Debug CleanPath

* Fix removed leading directory separator char in Linux breaks file path

Remove debugging statements

* Move combine to PathUtils

* PathUtils + PathUtilsTests

* Remove replicated (3x) tests throughout ResponseWithBodyFromFileTests

Co-authored-by: Stef Heyenrath <Stef.Heyenrath@gmail.com>

src/WireMock.Net/Util/PathUtils.cs

@@ -0,0 +1,36 @@
+using System.IO;
+
+namespace WireMock.Util
+{
+    internal static class PathUtils
+    {
+        /// <summary>
+        /// Robust handling of the user defined path.
+        /// Also supports Unix and Windows platforms
+        /// </summary>
+        /// <param name="path">The path to clean</param>
+        public static string CleanPath(string path)
+        {
+            return path?.Replace('/', Path.DirectorySeparatorChar).Replace('\\', Path.DirectorySeparatorChar);
+        }
+
+        /// <summary>
+        /// Removes leading directory separator chars from the filepath, which could break Path.Combine
+        /// </summary>
+        /// <param name="path">The path to remove the loading DirectorySeparatorChars</param>
+        public static string RemoveLeadingDirectorySeparators(string path)
+        {
+            return path?.TrimStart(new[] { Path.DirectorySeparatorChar });
+        }
+
+        /// <summary>
+        /// Combine two paths
+        /// </summary>
+        /// <param name="root">The root path</param>
+        /// <param name="path">The path</param>
+        public static string Combine(string root, string path)
+        {
+            return Path.Combine(root, RemoveLeadingDirectorySeparators(path));
+        }
+    }
+}