(#126) UsernamePasswordLogin: NPE when login() multiple times

[llorllale]

(FIX) UsernamePasswordLogin.login() was erroneously resetting user/pass
          fields to null. This attributes are now declared final.

closes #126

[codecov]

Codecov Report

Merging #127 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage    90.8%   90.74%   -0.06%     
==========================================
  Files          47       47              
  Lines         685      681       -4     
  Branches       32       32              
==========================================
- Hits          622      618       -4     
  Misses         55       55              
  Partials        8        8

Impacted Files	Coverage Δ
...le/youtrack/api/session/UsernamePasswordLogin.java	93.1% <100%> (-0.84%)	⬇️

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