Cosmetic issue: Keep insertion order of attributes of Head contribution elements

The HeadElement implementation [1] doesn't keep the insertion order for
attributes.

for example, the following can happen:
<meta content="..." name="...">
goes to
<meta name="..." content="...">

1: http://svn.onehippo.org/repos/hippo/hippo-cms7/site-toolkit/trunk/commons/src/main/java/org/hippoecm/hst/core/component/HeadElementImpl.java