title: Error occur if I use base class to define a key
body: <p>I get an error for define <code>DbGeography</code> in EF6 (i don't have this Error in EF5) entity type if I use it from base class to define key:
  for this class :</p>
  
  <pre><code>public class University
  {
      public int UniversityID { get; set; }
      public string Name { get; set; }
      public DbGeography Location { get; set; }
  }
  </code></pre>
  
  <p>university class mapping:</p>
  
  <pre><code>class UniversityMap : EntityTypeConfiguration&lt;University&gt;
  {
      public UniversityMap()
      {
          this.ToTable("University");
          this.HasKey(u =&gt; u.UniversityID);
  
      }
  }
  </code></pre>
  
  <p>everything is correct and EntityFramework does it correctly, but if I change class to :</p>
  
  <pre><code>public class BaseClass
  {
      public int UniversityID { get; set; }
  }
  public class University:BaseClass
  {
      public string Name { get; set; }
      public DbGeography Location { get; set; }
  }
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
    <p>One or more validation errors were detected during model generation:</p>
    
    <p>Data.DbGeography: : EntityType 'DbGeography' has no key
    defined. Define the key for this EntityType.
    DbGeographies: EntityType: EntitySet 'DbGeographies' is based on type 'DbGeography' that has no keys defined. </p>
  </blockquote>
  
  <p>!!!???
  Thanks</p>