FIX - nested non-rasdaman element is not escaped properly when updating coverage's metadata

[Bang Pham Huu]

For example, this metadata file has:

<ras:rasdamanCoverageMetadata>
    <ras:catalog>
       <ras:title>Arctic bathymetry type identifiers</ras:title>
       <ras:description>
           Test123 <a href="htt://abc">test</a> Test345
       </ras:description>
       <ras:ourTerms />
    </ras:catalog>
</ras:rasdamanCoverageMetadata>

here <a> element is non-rasdaman metadata element and petascope only escapes < character for the opening tag element, but not closing tag element, e.g. current escaped result looks like this:

       <ras:description>
           Test123 !ST;a href="htt://abc"!ET;test</a> Test345
       </ras:description>