allow coverageType = NULL

[kvanmalssen]

If the coverageType element is used, in the XML expression it must be Spatial or Temporal. Need to be able to supress coverageType element when coverageType is not used be coverage is.

if coverage="a" and coverageType="Spatial" then

<pbcoreCoverage>
<coverage>a</coverage>
<coverageType>Spatial</coverageType>
</pbcoreCoverage>

if coverage="a" and coverageType="" then

<pbcoreCoverage>
<coverage>a</coverage>
</pbcoreCoverage>

but
<pbcoreCoverage>
<coverage>a</coverage>
<coverageType></coverageType>
</pbcoreCoverage>
or
<pbcoreCoverage>
<coverage>a</coverage>
<coverageType/>
</pbcoreCoverage>