exempt capabilities from error 1006

Added two unit tests.

ArrowSetup errorCheck should exempt capabilities from error 1006 if its
value is null or empty or undefined

ArrowSetup errorCheck should exempt arguments if
config.error1006.exempt is specified
do not modify existing settings if not specified in config string.

Add one more unit test,

ArrowSetup errorCheck should exempt arguments if
config.error1006.exempt is specified but keep other existing exemption
settings if it is not specified in config string

For example, by default capabilities is exempted.
When user only provide a config string which does not modify the
capabilities, the existing exemption settings of capabilities should be
preserved.
Merge pull request #142 from bowdev/null_argument

exempt capabilities from error 1006