Fix for not always finding log4net, reported in https://nhibernate.jira.com/browse/NH-2821

Instead of checking for file on disk (which might fail, for example when
using ShadowCopy, like when running inside NUnit), check if log4net
assembly is loaded in current AppDomain.

Author: whut

src/NHibernate/Logging.cs

@@ -87,7 +87,7 @@ private static string GetNhibernateLoggerClass()
 				string binPath = relativeSearchPath == null ? baseDir : Path.Combine(baseDir, relativeSearchPath);
 				string log4NetDllPath = binPath == null ? "log4net.dll" : Path.Combine(binPath, "log4net.dll");
 
-				if (File.Exists(log4NetDllPath))
+				if (File.Exists(log4NetDllPath) || AppDomain.CurrentDomain.GetAssemblies().Any(a => a.GetName().Name == "log4net"))
 				{
 					nhibernateLoggerClass = typeof (Log4NetLoggerFactory).AssemblyQualifiedName;
 				}