c16:uniqueID element has wrong case

[tomjebo]

Describe the bug When writing out the c16:uniqueId element using the UniqueID class

<c:ser>
    ...
    <c:extLst>
        <c:ext xmlns:c16="http://schemas.microsoft.com/office/drawing/2014/chart"
               uri="{C3380CC4-5D6E-409C-BE32-E72D297353CC}">
            <c16:uniqueID val="{00000000-2153-43F5-A17B-C2679033F518}" />
        </c:ext>
    </c:extLst>
</c:ser>

Observed behavior c16:uniqueID is written

Expected behavior c16:uniqueId should be written

Desktop (please complete the following information):

• DocumentFormat.OpenXml Version: 3.0.2