Fix enums used as parameter for string columns (#2622)

Special treatment of enums mapped on string columns because all reasons
for checking string lengths or streamlining the length of parameters of
queries on string columns to the length of the column for SQL Server
are also valid for enums because they are sometimes by convention used
to model the possible values of a string column

* Fixes #2621

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Fixture.cs

@@ -0,0 +1,20 @@
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH2621Enum
+{
+	public class Fixture : BugTestCase
+	{
+		[Test]
+		public void TestOk()
+		{
+			using (var s = OpenSession())
+			using (s.BeginTransaction())
+			{
+				var query = s.CreateQuery(
+					@"SELECT Name FROM NHibernate.Test.NHSpecificTest.GH2621Enum.ClassWithString ROOT WHERE ROOT.Kind = :kind");
+				query.SetParameter("kind", Kind.SomeKind);
+				Assert.DoesNotThrow(() => query.List());
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Mappings.hbm.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
+           namespace="NHibernate.Test.NHSpecificTest.GH2621Enum"
+           assembly="NHibernate.Test"
+>
+  <class name="ClassWithString" >
+    <id name="Id">
+      <generator class="increment"/>
+    </id>
+    <property name="Name"/>
+    <property name="Kind"/>
+  </class>
+
+</hibernate-mapping>

src/NHibernate.Test/NHSpecificTest/GH2621Enum/Model.cs

@@ -0,0 +1,15 @@
+namespace NHibernate.Test.NHSpecificTest.GH2621Enum
+{
+	public abstract class ClassWithString
+	{
+		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual string Kind { get; set; }
+	}
+
+	public enum Kind 
+	{
+		SomeKind,
+		SomeOtherKind
+	}
+}

src/NHibernate/Type/AbstractStringType.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Data.Common;
 using System.Globalization;
@@ -58,6 +58,9 @@ public override void Set(DbCommand cmd, object value, int index, ISessionImpleme
 		{
 			var parameter = cmd.Parameters[index];
 
+			if (value is Enum)
+				value = value.ToString();
+
 			//Allow the driver to adjust the parameter for the value
 			session.Factory.ConnectionProvider.Driver.AdjustParameterForValue(parameter, SqlType, value);