Allow option to preserve Extended Attributes on patron import update.

Allow option to preserve Extended Attributes on patron import update.

Essentially, this patch provides the option to overwrite only matching
Extended Attributes, instead of all of them, treating the ext. fields more
like normal fields.

Several functions added to Members::Attributes with corresponding tests.

[ LL ref. 342 ]

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>