Make `null` keyword editable

[cyilcode]

Currently, DynamicQueryAttribute converts null keyword into actual null searches. However, this might be a blocking issue for some users that uses string keyword null in their datasets.

For example this:
?o=Equals&p=MyProperty&v=null

will be converted as:
SELECT * FROM MyTable where MyProperty is null

however the actualy user intention might be
SELECT * FROM MyTable where MyProperty = 'null'

Solution:

Make null conversion keyword an editable option in DynamicQueryBuilderSettings object.