Avoid re-entrency in HTTPCredidential::fetch

It is likely to re-enter if there is two jobs that asks for a password.

Example:
 1. log out
 2. restart the application
 3. enter a wrong password
 4. enter a wrong password again a few times
 5. enter the correct password
 6. it should must not prompt for the password again.

Because of the re-entrency, it was still prompting for the password several
times after the right password had been entered