Allow Microsoft.OData.Client.Serializer.GetKeyString to receive an IDictionary Object

[KenitoInc]

When working on a fix on issue OData/lab#96 I realized that OData Client's Serializer.GetKeyString receives a DataServiceContext Object and a Concrete Dictionary instead of an IDictionary. Since Dictionary implements IDictionary, we can update the method as follows
public static string GetKeyString(DataServiceContext context, IDictionary<string, object> keys)
{
// Implementation
}

Assemblies affected

Microsoft.OData.Client v7.6.2

Reproduce steps

Pass an IDictionary in the ByKey methods of OData Connected Service.

Expected result

No error is thrown

Actual result

This error is thrown: cannot convert from 'System.Collections.Generic.IDictionary<string, object>' to 'System.Collections.Generic.Dictionary<string, object>'

[xuzhg]

Merged and close it.