SEC-2321: Refine to use X-Requested-With: XMLHttpRequest
SEC-2358: Add RequestHEaderRequestMatcher#toString()
SEC-1932 Added PBKDF2PasswordEncoder.

 - Also moved some logic into a new class, AbstractPasswordEncoder. Both PBKDF2PasswordEncoder and the now-simplified StandardPasswordEncoder extend AbstractPasswordEncoder.
 - Added tests for PBKDF2PasswordEncoder .