Improvement for LINE provider (#582)

Fix issue of login page being ignored if the user did not log in with LINE first.

Author: linmasaki

src/AspNet.Security.OAuth.Line/LineAuthenticationHandler.cs

@@ -40,7 +40,7 @@ protected override string BuildChallengeUrl([NotNull] AuthenticationProperties p
                 ["redirect_uri"] = redirectUri,
                 ["state"] = Options.StateDataFormat.Protect(properties),
                 ["scope"] = FormatScope(),
-                ["prompt"] = Options.Prompt ? "consent" : "none"
+                ["prompt"] = Options.Prompt ? "consent" : string.Empty
             });
 
         protected override async Task<OAuthTokenResponse> ExchangeCodeAsync([NotNull] OAuthCodeExchangeContext context)