can't update several fields, one by one only

e.g.

<Entity Type="test-instance"><Fields><Field Name="status"><Value>Passed</Value></Field><Field Name="exec-time"><Value>10:10:29</Value></Field></Fields></Entity>

 HTTP value return OK, but this xml update only status! but i can update status in one PUT method and than update exec-time, doing second POST.

Seem's strange why don't update two fields at one POST.

Please help!