Change getAuthBackends http method to GET.

Fixes: #494

Author: Sam1301

app/src/main/java/com/zulip/android/service/ZulipServices.java

@@ -50,7 +50,7 @@ Call<GetMessagesResponse> getMessages(@Query("anchor") String anchor,
     @PUT("v1/users/me/pointer")
     Call<ResponseBody> updatePointer(@Field("pointer") String pointer);
 
-    @POST("v1/get_auth_backends")
+    @GET("v1/get_auth_backends")
     Call<ZulipBackendResponse> getAuthBackends();
 
     @FormUrlEncoded