Make NameValueCollection.HasKeys() method virtual and override it in WebHeaderCollection

[madmir]

This suggestion is order to provide a fix for #27971.

AllKeys and Keys are overridden in WebHeaderCollection, but we are unable to change behavior of related HasKeys() method. This causes inconsistency described in aforementioned bug report.

InternalHasKeys(), as it's currently defined, seems to be redundant as well.

Current

public class NameValueCollection : NameObjectCollectionBase
{
    public bool HasKeys();
    internal virtual bool InternalHasKeys();
}
public class WebHeaderCollection : NameValueCollection, ISerializable
{
}

Proposed

public class NameValueCollection : NameObjectCollectionBase
{
    public virtual bool HasKeys();
}
public class WebHeaderCollection : NameValueCollection, ISerializable
{
    public override bool HasKeys();
}

[davidsh]

It might be a breaking change to remove a method such as this:

internal virtual bool InternalHasKeys();

from the NameValueCollection type. So, this API proposal would have to be reworked to not be breaking existing usage.

[karelz]

WebClient is legacy API and as such we are not thrilled to make API changes elsewhere to make it work better.
Let's primarily try to find other ways how to make WebClient work.