Fix the Dropbox provider to use POST instead of GET for userinfo retrieval

Author: cristiz1992

src/AspNet.Security.OAuth.Dropbox/DropboxAuthenticationHandler.cs

@@ -28,7 +28,7 @@ public DropboxAuthenticationHandler([NotNull] HttpClient client)
         protected override async Task<AuthenticationTicket> CreateTicketAsync([NotNull] ClaimsIdentity identity,
             [NotNull] AuthenticationProperties properties, [NotNull] OAuthTokenResponse tokens)
         {
-            var request = new HttpRequestMessage(HttpMethod.Get, Options.UserInformationEndpoint);
+            var request = new HttpRequestMessage(HttpMethod.Post, Options.UserInformationEndpoint);
             request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
             request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", tokens.AccessToken);