Add email claim for Yahoo provider (#488)

Email is a mandatory claim. Add it by default to claims.

Author: farajfarook

src/AspNet.Security.OAuth.Yahoo/YahooAuthenticationOptions.cs

@@ -27,6 +27,7 @@ public YahooAuthenticationOptions()
 
             ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "sub");
             ClaimActions.MapJsonKey(ClaimTypes.Name, "name");
+            ClaimActions.MapJsonKey(ClaimTypes.Email, "email");
             ClaimActions.MapJsonKey(Claims.FamilyName, "family_name");
             ClaimActions.MapJsonKey(Claims.GivenName, "given_name");
             ClaimActions.MapJsonKey(Claims.Picture, "picture");