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

Author: cristiz1992

samples/Mvc.Client/Startup.cs

@@ -42,6 +42,12 @@ public void ConfigureServices(IServiceCollection services)
                 options.ClientId = "49e302895d8b09ea5656";
                 options.ClientSecret = "98f1bf028608901e9df91d64ee61536fe562064b";
                 options.Scope.Add("user:email");
+            })
+
+            .AddDropbox(options =>
+            {
+                options.ClientId = "jpk24g2uxfxe939";
+                options.ClientSecret = "qbxvkjk5la7mjp6";
             });
 
             services.AddMvc();

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

@@ -32,7 +32,7 @@ public DropboxAuthenticationHandler(
         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);