[add] invariant language test

[r] language field name to constant

src/Simplify.Web.Tests/Modules/LanguageManagerTests.cs

@@ -193,4 +193,17 @@ public void Constructor_NoHeaderLanguage_DefaultLanguageSet()
 		// Assert
 		Assert.AreEqual("en", _languageManager.Language);
 	}
+
+	[Test]
+	public void Constructor_BadDefaultLanguage_InvariantLanguageSet()
+	{
+		// Arrange
+		_settings.SetupGet(x => x.DefaultLanguage).Returns("badlanguage");
+
+		// Act
+		_languageManager = new LanguageManager(_settings.Object, _context.Object);
+
+		// Assert
+		Assert.AreEqual("iv", _languageManager.Language);
+	}
 }

src/Simplify.Web/Modules/LanguageManager.cs

@@ -16,6 +16,11 @@ public class LanguageManager : ILanguageManager
 	/// </summary>
 	public const string CookieLanguageFieldName = "language";
 
+	/// <summary>
+	/// Language field name in request header
+	/// </summary>
+	public const string HeaderLanguageFieldName = "Accept-Language";
+
 	private readonly IResponseCookies _responseCookies;
 
 	/// <summary>
@@ -32,7 +37,7 @@ public LanguageManager(ISimplifyWebSettings settings, HttpContext context)
 
 		if (!settings.AcceptHeaderLanguage || (settings.AcceptHeaderLanguage && !TrySetLanguageFromRequestHeader(context)))
 			if (!SetCurrentLanguage(settings.DefaultLanguage))
-				Language = "iv";
+				SetCurrentLanguage("iv");
 	}
 
 	/// <summary>
@@ -87,7 +92,7 @@ private bool TrySetLanguageFromCookie(HttpContext context)
 
 	private bool TrySetLanguageFromRequestHeader(HttpContext context)
 	{
-		var languages = context.Request.Headers["Accept-Language"];
+		var languages = context.Request.Headers[HeaderLanguageFieldName];
 
 		if (languages.Count == 0)
 			return false;