Add XML declaration to POM's

All POM's within the Weld project are without a XML declaration like:

<?xml version="1.0" encoding="UTF-8"?>

It would be good to add it.
This causes problems with Nexus3 which does not mirror Weld artifacts when "Strict Content Type Validation" is enabled on a repository which is the default.

org.sonatype.nexus.repository.InvalidContentException: Detected content type [text/html], but expected [application/xml, text/xml, application/x-xml]: org/jboss/weld/weld-core-parent/2.2.1.Final/weld-core-parent-2.2.1.Final.pom.xml

The POM is correctly loaded from the upstream repository with content-type application/xml, but the mime type detection of Nexus 3 inspects the POM and concludes it is not an XML document due to the missing XML declaration.
See nexus.log for the detailed logging.