Dimension unit not stored unless changed from the default

[ddeboer]

The unit of the dimension is only changed if it is changed from the default (e.g. from mm to cm). If you then change the unit back to mm, the unit is displayed.

So

produces:

<#object-f04c43a1-39d3-4ae2-a209-01679a3bd62f-depth> a <http://schema.org/QuantitativeValue>;
    <http://schema.org/value> 1234.5.

While

produces:

<#object-f04c43a1-39d3-4ae2-a209-01679a3bd62f-depth> a <http://schema.org/QuantitativeValue>;
    <http://schema.org/unitCode> "CMT";
    <http://schema.org/value> 1234.5.

Possibly a side-effect of #548.