fixing the falsy attaching "invalid" attribute names

if an attribute mapping was set to false, it's vCard name got added to the value of the previous valid mapping.
Now any line from the vCardString.split('\n') array that does start with a key from the mapping object where the value from mapping is not a string representation of the JSONAttributeName (length > 0) will be ignored (not even added to the value of the previous positive match, which is a feature to concatenate values containing \n values back together)
fixing the falsy concatenation of invalid attribute names

if an attribute mapping was set to false, it's vCard name got added to the value of the previous valid mapping.
Now any line from the vCardString.split('\n') array that does start with a key from the mapping object w