Improve testing coverage

[josecelano]

Current coverage report:

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
de.rs                             190                60    68.42%          36                 9    75.00%         237                43    81.86%           0                 0         -
error.rs                           31                31     0.00%          12                12     0.00%          51                51     0.00%           0                 0         -
ser.rs                            118                43    63.56%          57                13    77.19%         277                59    78.70%           0                 0         -
ser/string.rs                      30                28     6.67%          30                28     6.67%         125               119     4.80%           0                 0         -
value.rs                          174                13    92.53%         112                 5    95.54%         502                19    96.22%           0                 0         -
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             543               175    67.77%         247                67    72.87%        1192               291    75.59%           0                 0         -