Message#setHeaderPart should overwrite any previous value

org.apache.ode.bpel.iapi.Message#setHeaderPart should overwrite any previous value if called twice.

As of today, a new element with the same name is appended to the document. So you can get something like:

<?xml version="1.0" encoding="UTF-8"?>
<header>
<header_ABC>foo</header_ABC>
<header_ABC>bar</header_ABC>
</header>

Then when Message.getHeaderPart is invoked, the first element is returned.

The fix must make sure that Message#setHeaderPart overwrite any existing value of the given header.