Fix bug with environment name beeng overwritten

After being set using UseEnvironment

src/Microsoft.AspNet.Hosting/WebHostBuilder.cs

@@ -27,6 +27,7 @@ namespace Microsoft.AspNet.Hosting
         private readonly WebHostOptions _options;
 
         private Action<IServiceCollection> _configureServices;
+        private string _environmentName;
 
         // Only one of these should be set
         private StartupMethods _startup;
@@ -127,6 +128,10 @@ namespace Microsoft.AspNet.Hosting
             var startupLoader = hostingContainer.GetRequiredService<IStartupLoader>();
 
             _hostingEnvironment.Initialize(appEnvironment.ApplicationBasePath, _options);
+            if (!string.IsNullOrEmpty(_environmentName))
+            {
+                _hostingEnvironment.EnvironmentName = _environmentName;
+            }
             var engine = new HostingEngine(hostingServices, startupLoader, _options, _config, _captureStartupErrors);
 
             // Only one of these should be set, but they are used in priority
@@ -155,7 +160,7 @@ namespace Microsoft.AspNet.Hosting
                 throw new ArgumentNullException(nameof(environment));
             }
 
-            _hostingEnvironment.EnvironmentName = environment;
+            _environmentName = environment;
             return this;
         }
 

test/Microsoft.AspNet.Hosting.Tests/WebHostBuilderTests.cs

@@ -124,6 +124,23 @@ namespace Microsoft.AspNet.Hosting
             }
         }
 
+        [Fact]
+        public void UseEnvironmentIsNotOverriden()
+        {
+            var vals = new Dictionary<string, string>
+            {
+                { "ENV", "Dev" },
+            };
+            var builder = new ConfigurationBuilder()
+                .AddInMemoryCollection(vals);
+            var config = builder.Build();
+
+            var expected = "MY_TEST_ENVIRONMENT";
+            var webHost = new WebHostBuilder(config, captureStartupErrors: true).UseEnvironment(expected).Build();
+
+            Assert.Equal(expected, webHost.ApplicationServices.GetService<IHostingEnvironment>().EnvironmentName);
+        }
+
         private WebHostBuilder CreateWebHostBuilder()
         {
             var vals = new Dictionary<string, string>