inconsistent "defaults" in Url()

[marksantcroos]

>>> u = radical.utils.Url('/Users/mark/pretty_picture_of_andre.png')
>>> u.schema
''
>>> u.host
>>>

[andre-merzky]

>>> import radical.utils as ru
>>> u = ru.Url ('/etc/passwd')
>>> u.host
>>> u.schema
>>> 

[andre-merzky]

I had to switch the default from None to empty string -- the urlparser we use depends on non-existing elements being empty strings, and will otherwise raise exceptions in several places. Fixing that behavior doesn't seem easy, so as long as we are stuck with that parser, strings it is. Note that the default for port remains None -- this is an int, and zero as value does not make sense, and triggers the inclusion of the zero port in all URL serializations...