Fix #365 - Make IFormFileCollection implmenent IReadOnlyList<T>

Author: rynowak

src/Microsoft.AspNet.Http.Abstractions/IFormFileCollection.cs

@@ -5,12 +5,12 @@
 
 namespace Microsoft.AspNet.Http
 {
-    public interface IFormFileCollection : IList<IFormFile>
+    public interface IFormFileCollection : IReadOnlyList<IFormFile>
     {
         IFormFile this[string name] { get; }
 
         IFormFile GetFile(string name);
 
-        IList<IFormFile> GetFiles(string name);
+        IReadOnlyList<IFormFile> GetFiles(string name);
     }
 }

src/Microsoft.AspNet.Http/FormFileCollection.cs

@@ -18,7 +18,7 @@ public IFormFile GetFile(string name)
             return Find(file => string.Equals(name, GetName(file.ContentDisposition)));
         }
 
-        public IList<IFormFile> GetFiles(string name)
+        public IReadOnlyList<IFormFile> GetFiles(string name)
         {
             return FindAll(file => string.Equals(name, GetName(file.ContentDisposition)));
         }