Prevent invalid Axiom attributes

Axiom 1.2.14 allows creation of attributes with a namespace URI and an
empty prefix, but the behavior for attributes with such an invalid name
is undefined. Axiom 1.2.15 will refuse to create such an attribute. This
change modifies AxiomSoapElement#addAttribute so that it lets Axiom
generate a prefix if necessary. All tests now pass successfully with
Axiom 1.2.15-SNAPSHOT.
Merge pull request #2 from veithen/master

* axiom_attribute:
  Prevent invalid Axiom attributes