Bring unit test code coverage to 90%

[rahulraghu94]

Current coverage:

 # code and logs here.
Name                                             Stmts   Miss  Cover   Missing
------------------------------------------------------------------------------
pymodbus/__init__.py                                12      4    67%   25-28
pymodbus/bit_read_message.py                        74      0   100%
pymodbus/bit_write_message.py                       97      0   100%
pymodbus/client/__init__.py                          0      0   100%
pymodbus/client/async.py                            73      1    99%   157
pymodbus/client/common.py                           39      0   100%
pymodbus/client/sync.py                            182      9    95%   344, 370-372, 379, 384-385, 389, 403
pymodbus/compat.py                                  44      3    93%   94-96
pymodbus/constants.py                               39      0   100%
pymodbus/datastore/__init__.py                       5      0   100%
pymodbus/datastore/context.py                       56      0   100%
pymodbus/datastore/database/__init__.py              3      0   100%
pymodbus/datastore/database/redis_datastore.py      86      0   100%
pymodbus/datastore/database/sql_datastore.py        67      1    99%   113
pymodbus/datastore/remote.py                        35      1    97%   84
pymodbus/datastore/store.py                         69      0   100%
pymodbus/device.py                                 164      0   100%
pymodbus/diag_message.py                           227      8    96%   72-74, 122, 725-729
pymodbus/events.py                                  60      0   100%
pymodbus/exceptions.py                              30      2    93%   91-92
pymodbus/factory.py                                 78      0   100%
pymodbus/file_message.py                           147      0   100%
pymodbus/interfaces.py                              46      0   100%
pymodbus/internal/__init__.py                        0      0   100%
pymodbus/internal/ptwisted.py                       20      3    85%   29, 33-34
pymodbus/mei_message.py                             75      1    99%   137
pymodbus/other_message.py                          148      0   100%
pymodbus/payload.py                                154      0   100%
pymodbus/pdu.py                                     73      0   100%
pymodbus/register_read_message.py                  129      2    98%   47, 297
pymodbus/register_write_message.py                 133      2    98%   72, 149
pymodbus/server/__init__.py                          0      0   100%
pymodbus/server/async.py                           134     52    61%   58-63, 70-84, 91-96, 163-168, 175-189, 197-201, 270
pymodbus/server/sync.py                            240      7    97%   62-65, 303-304, 457
pymodbus/transaction.py                            424    145    66%   71-74, 80-87, 92-103, 109-155, 158, 161-181, 193, 202, 209, 454, 458-459, 469, 471, 485-486, 492, 584-586, 597-598, 644, 674-689, 707-716, 722, 826-827, 857-867, 942, 1019-1029, 1056-1058, 1069-1070
pymodbus/utilities.py                               73      0   100%
pymodbus/version.py                                 16      1    94%   34
------------------------------------------------------------------------------
TOTAL                                             3252    242    93%

[dhoomakethu]

Thanks @rahulraghu94 , merged