batch() returns err on any null/undef key/value

fixes #19
LD_CB_ERR_IF_OPTION_NULL_OR_UNDEFINED -> LD_CB_ERR_IF_NULL_OR_UNDEFINED for args
use lower case names for properties in error messages

removed last parameter in LD_STRING_OR_BUFFER_TO_SLICE() and used
the first parameter in the error message instead
Merge pull request #20 from rvagg/batch-null-undefined

batch() returns err on any null/undef key/value