properly handle null attribute values in WFS update

[tschaub]

Currently, if a feature attribute value is null (e.g. {foo: null}), we write something like:

<Property>
    <Name>foo</Name>
    <Value></Value>
</Property>

This should look like the following:

<Property>
    <Name>foo</Name>
</Property>

An attribute with a null value should not show up in an insert. An attribute with an undefined value should not show up in insert or update.

[tschaub]

properly handle null and undefined attribute values

[tschaub]

Tests pass (in FF3). Thanks for the review.

[ahocevar]

Very nice! Tests also pass in IE7. Please commit.

[tschaub]

(In [9615]) Proper handling of null and undefined attribute values in WFS transactions. r=ahocevar (closes #2222)