Fix path trimming in .NET Core AspNetFileSystem

Trimming should handle forward as well as backslashes. There aren't any
unit tests for this already that I could find, so I verified manually by
calling:

  .AddScriptWithoutTransform("~/Content/*.js");

Within the .NET Core sample project.

Fixes reactjs#526

Author: dustinsoftware

src/React.AspNet.Middleware/AspNetFileSystem.cs

@@ -38,7 +38,7 @@ public override string MapPath(string relativePath)
 			{
 				return relativePath;
 			}
-			relativePath = relativePath.TrimStart('~').TrimStart('/');
+			relativePath = relativePath.TrimStart('~').TrimStart('/').TrimStart('\\');
 
 			return Path.GetFullPath(Path.Combine(_hostingEnv.WebRootPath, relativePath));
 		}

src/React.Core/FileSystemBase.cs

@@ -30,7 +30,7 @@ abstract public class FileSystemBase : IFileSystem
 		public abstract string MapPath(string relativePath);
 
 		/// <summary>
-		/// Converts a path from a full filesystem path to anan application relative path (~/...)
+		/// Converts a path from a full filesystem path to an application relative path (~/...)
 		/// </summary>
 		/// <param name="absolutePath">Full path of the file</param>
 		/// <returns>App-relative path of the file</returns>