Feature/transform headers (#204)

* New feature that lets a user do find and replace on an upstream header

* can transform downstream and upstream headers, not sure if interface is good

* can replace location header with placeholder

* added some syntax

test/Ocelot.UnitTests/Headers/HttpResponseHeaderReplacerTests.cs

@@ -0,0 +1,91 @@
+using Xunit;
+using Shouldly;
+using TestStack.BDDfy;
+using System.Net.Http;
+using Ocelot.Headers;
+using Ocelot.Configuration;
+using System.Collections.Generic;
+using Ocelot.Responses;
+using System.Linq;
+
+namespace Ocelot.UnitTests.Headers
+{
+    public class HttpResponseHeaderReplacerTests
+    {
+        private HttpResponseMessage _response;
+        private HttpResponseHeaderReplacer _replacer;
+        private List<HeaderFindAndReplace> _headerFindAndReplaces;
+        private Response _result;
+
+        public HttpResponseHeaderReplacerTests()
+        {
+            _replacer = new HttpResponseHeaderReplacer();
+        }
+        [Fact]
+        public void should_replace_headers()
+        {
+            var response = new HttpResponseMessage();
+            response.Headers.Add("test", "test");
+
+            var fAndRs = new List<HeaderFindAndReplace>();
+            fAndRs.Add(new HeaderFindAndReplace("test", "test", "chiken", 0));
+
+            this.Given(x => GivenTheHttpResponse(response))
+                .And(x => GivenTheFollowingHeaderReplacements(fAndRs))
+                .When(x => WhenICallTheReplacer())
+                .Then(x => ThenTheHeadersAreReplaced())
+                .BDDfy();
+        }
+
+        [Fact]
+        public void should_not_replace_headers()
+        {
+            var response = new HttpResponseMessage();
+            response.Headers.Add("test", "test");
+
+            var fAndRs = new List<HeaderFindAndReplace>();
+
+            this.Given(x => GivenTheHttpResponse(response))
+                .And(x => GivenTheFollowingHeaderReplacements(fAndRs))
+                .When(x => WhenICallTheReplacer())
+                .Then(x => ThenTheHeadersAreNotReplaced())
+                .BDDfy();
+        }
+
+
+        private void ThenTheHeadersAreNotReplaced()
+        {
+             _result.ShouldBeOfType<OkResponse>();
+            foreach (var f in _headerFindAndReplaces)
+            {
+                _response.Headers.TryGetValues(f.Key, out var values);
+                values.ToList()[f.Index].ShouldBe("test");
+            }
+        }
+
+        private void GivenTheFollowingHeaderReplacements(List<HeaderFindAndReplace> fAndRs)
+        {
+            _headerFindAndReplaces = fAndRs;
+        }
+
+        private void GivenTheHttpResponse(HttpResponseMessage response)
+        {
+            _response = response;
+        }
+
+        private void WhenICallTheReplacer()
+        {
+            _result = _replacer.Replace(_response, _headerFindAndReplaces);
+        }
+
+         private void ThenTheHeadersAreReplaced()
+        {
+            _result.ShouldBeOfType<OkResponse>();
+            foreach (var f in _headerFindAndReplaces)
+            {
+                _response.Headers.TryGetValues(f.Key, out var values);
+                values.ToList()[f.Index].ShouldBe(f.Replace);
+            }
+        }
+    }
+}