Improve mechanism to provide record used for harvest operation

[dstenger]

See #8 (comment).

[etj]

I copied the sample metadata in our dgiwg test server at
http://dgiwg.geo-solutions.it/test/metadata/DMFMetadataRecord.xml
so that the returned content-type is text/xml, and so it's accepted by gn.

This is a temporary workaround to make the test pass.

[lgoltz]

We provided the record at https://opengeospatial.github.io/ets-cat20-dgiwg10/DMFMetadataRecord.xml, the returned content type is application/xml (default of ResourceFormat parameter of the harvest operation). The test issueHarvestRequest fails cause the service returns

<ows:ExceptionReport xmlns:ows="http://www.opengis.net/ows"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        version="1.2.0"
        xsi:schemaLocation="http://www.opengis.net/ows http://schemas.opengis.net/ows/1.0.0/owsExceptionReport.xsd">
  <ows:Exception exceptionCode="NoApplicableCode">
      <ows:ExceptionText>Host name 'opengeospatial.github.io' does not match the certificate subject provided by the peer (CN=www.github.com, O="GitHub, Inc.", L=San Francisco, ST=California, C=US)</ows:ExceptionText>
  </ows:Exception>
</ows:ExceptionReport>

@etj Can you take a look at this?