Fix issue with file permissions and include unit test for it

Author: geoperez

src/Unosquare.Labs.EmbedIO/Modules/StaticFilesModule.cs

@@ -240,7 +240,7 @@ private bool HandleGet(HttpListenerContext context, WebServer server, bool sendB
 
                 if (sendBuffer)
                 {
-                    buffer = new FileStream(localPath, FileMode.Open, FileAccess.Read, FileShare.Read);
+                    buffer = new FileStream(localPath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
 
                     if (usingPartial == false)
                     {

test/Unosquare.Labs.EmbedIO.Tests/StaticFilesModuleTest.cs

@@ -317,6 +317,30 @@ public async Task HeadIndex()
             }
         }
 
+        [Test]
+        public async Task FileWritable()
+        {
+            var endpoint = Resources.GetServerAddress();
+            var root = Path.GetTempPath();
+            var file = Path.Combine(root, "index.html");
+            File.WriteAllText(file, Resources.Index);
+
+            using (var server = new WebServer(endpoint, Logger))
+            {
+                server.RegisterModule(new StaticFilesModule(root));
+                server.RunAsync();
+
+                var webClient = new HttpClient();
+                var remoteFile = await webClient.GetStringAsync(endpoint);
+                File.WriteAllText(file, Resources.SubIndex);
+                var remoteUpdatedFile = await webClient.GetStringAsync(endpoint);
+                File.WriteAllText(file, nameof(WebServer));
+
+                Assert.AreEqual(Resources.Index, remoteFile);
+                Assert.AreEqual(Resources.SubIndex, remoteUpdatedFile);
+            }
+        }
+
         [TearDown]
         public void Kill()
         {

test/Unosquare.Labs.EmbedIO.Tests/TestObjects/Resources.cs

@@ -5,7 +5,8 @@ namespace Unosquare.Labs.EmbedIO.Tests.TestObjects
     public static class Resources
     {
         private const string ServerAddress = "http://localhost:{0}/";
-        public static int Counter = 9699;        
+        public static int Counter = 9699;    
+            
         public static string GetServerAddress(){
             Interlocked.Increment(ref Counter);
             return string.Format(ServerAddress, Counter);