FIX - adding external metadata to coverage returned wrong value in DescribeCoverage

[Bang Pham Huu]

For example, a coverage with empty metadata:

<ras:covMetadata xmlns:ras="http://www.rasdaman.org">
</ras:covMetadata>

then, updating coverage metadata with this content

<ras:rasdamanCoverageMetadata>
    <ras:catalog>
       ...
       <ras:ourTerms />
    </ras:catalog>
</ras:rasdamanCoverageMetadata>    

will return duplicated elements for <ras:rasdamanCoverageMetadata> in DescribeCoverage response.