dowload.pp: Quote the URL to get the data from

When URLs contains not only alphabetical characters like '&' the
URL parsed is not the one expected.

For example the following URL needs to be quoted to work as expected
'http://downloads.sourceforge.net/project/pentaho/Business%20Intelligence%20Server/5.1/biserver-ce-5.1.0.0-752.zip?r=&ts=1410402175&use_mirror=iweb'