remapNamespacePrefix: Preserve prefix of attribute names (#255)

Preserve prefix of attribute names when copying them over to the new
node.  This fixes an unintentional rewrite of `xml:space` to `space`
that also caused scour to strip whitespace that should have been
preserved.

Closes: #239
Signed-off-by: Niels Thykier <niels@thykier.net>

unittests/redundant-svg-namespace.svg

@@ -5,4 +5,5 @@
 		<title>Test</title>
 	</svg:rect>
 	<vector:rect height="100" width="100"/>
+	<svg:text xml:space="preserve">  Hallo World  </svg:text>
 </svg>