Add unit tests (98% line coverage and 1 bugfix)

[alexcasalboni]

Before:

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
lib.rs                             77                77     0.00%          10                10     0.00%         114               114     0.00%           0                 0         -
segment_payload.rs                 96                96     0.00%           2                 2     0.00%         119               119     0.00%           0                 0         -
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             173               173     0.00%          12                12     0.00%         233               233     0.00%           0                 0         -

After:

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
lib.rs                            142                13    90.85%          42                 3    92.86%         616                 7    98.86%           0                 0         -
segment_payload.rs                 89                15    83.15%           2                 0   100.00%         114                 2    98.25%           0                 0         -
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             231                28    87.88%          44                 3    93.18%         730                 9    98.77%           0                 0         -

Note: I also fixed a potential bug in case you pass an empty PageData object in the Context (see page_without_context_page_data).