Fix - update coverage's metadata with extra local metadata not persisted properly

[Bang Pham Huu]

This metadata below doesn't persist completely for section <TEST> in petascopedb when updating a coverage's metadata from file:

    <slices>
                        <slice>
                            <boundedBy>
                                <Envelope>
                                    <axisLabels>Lat Long</axisLabels>
                                    <srsDimension>2</srsDimension>
                                    <lowerCorner>-44.975 111.975</lowerCorner>
                                    <upperCorner>-8.975 130.975</upperCorner>
                                </Envelope>
                            </boundedBy>
                            <local_metadata_key>file1</local_metadata_key>
                            <TEST>
                                <metadaten>
                                    <Land>Land1</Land>
                                </metadaten>
                            </TEST>
                            <fileReferenceHistory>test_metadata_4515575.tiff</fileReferenceHistory>
                        </slice>
                        <slice>
                            <boundedBy>
                                <Envelope>
                                    <axisLabels>Lat Long</axisLabels>
                                    <srsDimension>2</srsDimension>
                                    <lowerCorner>-44.975 129.975</lowerCorner>
                                    <upperCorner>-8.975 135.975</upperCorner>
                                </Envelope>
                            </boundedBy>
                            <local_metadata_key>test_metadata_4515576</local_metadata_key>
                            <TEST>
                                <metadaten>
                                    <Land>Land2</Land>
                                </metadaten>
                            </TEST>
                            <fileReferenceHistory>test_metadata_4515576.tiff</fileReferenceHistory>
                        </slice>
                        <slice>
                            <boundedBy>
                                <Envelope>
                                    <axisLabels>Lat Long</axisLabels>
                                    <srsDimension>2</srsDimension>
                                    <lowerCorner>-44.975 134.975</lowerCorner>
                                    <upperCorner>-8.975 145.975</upperCorner>
                                </Envelope>
                            </boundedBy>
                            <local_metadata_key>test_metadata_4515577</local_metadata_key>
                            <TEST>
                                <metadaten>
                                    <Land>Land3</Land>
                                </metadaten>
                            </TEST>
                            <fileReferenceHistory>test_metadata_4515577.tiff</fileReferenceHistory>
                        </slice>
                        <slice>
                            <boundedBy>
                                <Envelope>
                                    <axisLabels>Lat Long</axisLabels>
                                    <srsDimension>2</srsDimension>
                                    <lowerCorner>-44.975 144.975</lowerCorner>
                                    <upperCorner>-8.975 155.975</upperCorner>
                                </Envelope>
                            </boundedBy>
                            <local_metadata_key>test_metadata_4525572</local_metadata_key>
                            <TEST>
                                <metadaten>
                                      <Land>Land4</Land>
                                </metadaten>
                            </TEST>
                            <fileReferenceHistory>test_metadata_4525572.tiff</fileReferenceHistory>
                        </slice>
                    </slices>
                    <Data_provider>Test</Data_provider>