Issue description: 

* I need to fix an issue with resolving of standard HTML entitities.
* I've implemented HtmlEntityReader - implementation of XmlReader which has a code to resolve entities
* Public API of our system provides a methods with usage of XmlReader, so user can pass XmlReader created using one of the XmlReader.Create methods

Current code of my xml unit tests is below:

```
using System.Xml;
using NUnit.Framework;

namespace Tests
{
    [TestFixture]
    public class XmlTests
    {
        // this test works
        [Test]
        public void TestEntitiesResolving1()
        {
            var path = QA.ResolvePath(@"html\bugs\317.html");
            using (var reader = new XmlTextReader(path, new NameTable()))
            {
                reader.XmlResolver = null; //to prevent DTD downloading
                var wrapper = new HtmlEntityReader(reader, XmlUtils.HtmlEntities);
                while (wrapper.Read()) { }
            }
        }

        // this test does not work - why?
        // what's the difference in initialization of internal XmlTextReaderImpl?
        [Test]
        public void TestEntitiesResolving2()
        {
            var path = QA.ResolvePath(@"html\bugs\317.html");
            var settings = new XmlReaderSettings
                           {
                               XmlResolver = null, //to prevent DTD downloading
                               NameTable = new NameTable(),
                               ProhibitDtd = false,
                               CheckCharacters = false,
                           };
            using (var reader = XmlReader.Create(path, settings))
            {
                var wrapper = new HtmlEntityReader(reader, XmlUtils.HtmlEntities);
                while (wrapper.Read()) { }
            }
        }
    }
}

```

Partial code of HtmlEntityReader is below:

```
internal sealed class HtmlEntityReader : XmlReader
{
    readonly XmlReader _impl;
    readonly Hashtable _entitySet;
    string _entityValue;

    public HtmlEntityReader(XmlReader reader, Hashtable entitySet)
    {
        if (reader == null) throw new ArgumentNullException("reader");
        if (entitySet == null) throw new ArgumentNullException("entitySet");
        _impl = reader;
        _entitySet = entitySet;
    }

    public override XmlNodeType NodeType
    {
        get { return _entityValue != null ? XmlNodeType.Text : _impl.NodeType; }
    }

    public override string LocalName
    {
        get { return _entityValue != null ? string.Empty : _impl.LocalName; }
    }

    public override string Prefix
    {
        get { return _entityValue != null ? string.Empty : _impl.Prefix; }
    }

    public override string Name
    {
        get { return _entityValue != null ? string.Empty : _impl.Name; }
    }

    public override bool HasValue
    {
        get { return _entityValue != null || _impl.HasValue; }
    }

    public override string Value
    {
        get { return _entityValue ?? _impl.Value; }
    }

    public override bool CanResolveEntity
    {
        get { return true; }
    }

    public override void ResolveEntity()
    {
        //it seems this does not call - why?
    }

    public override bool Read()
    {
        _entityValue = null;
        if (!_impl.Read()) return false;
        if (NodeType == XmlNodeType.EntityReference)
        {
           //resolving of entity reference
           _entityValue = (string)_entitySet[Name];
        }
        return true;
    }

    // ... delegation of XmlReader abstract methods to _impl
}

```

I've got the exception:

```

System.Xml.XmlException: Reference to undeclared entity 'nbsp'. Line 4, position 5.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.Throw(String res, String arg, Int32 lineNo, Int32 linePos)
at System.Xml.XmlTextReaderImpl.HandleGeneralEntityReference(String name, Boolean isInAttributeValue, Boolean pushFakeEntityIfNullResolver, Int32 entityStartLinePos)
at System.Xml.XmlTextReaderImpl.HandleEntityReference(Boolean isInAttributeValue, EntityExpandType expandType, ref Int32 charRefEndPos)
at System.Xml.XmlTextReaderImpl.ParseText(ref Int32 startPos, ref Int32 endPos, ref Int32 outOrChars)
at System.Xml.XmlTextReaderImpl.ParseText()
at System.Xml.XmlTextReaderImpl.ParseElementContent()
at System.Xml.XmlTextReaderImpl.Read()
... private staff

```

Could you provide a quick advice or link to a solution while I am fixing / investigating / searching this issue through my own efforts?