xml:space attribute changes need document reload

First reported in Bug #275184 "Can't type spaces in imported PDF text " (see comment #7).

Changing the xml:space attribute value in the XML editor doesn't update the text on canvas.

Steps to reproduce:
1. Create a text (two words with a space between, such as "test test").
2. Open the XML editor and replace the xml:space value with "default" (removing the attribute also works).
3. Select the Text tool and add spaces between the two text words.
-> Inkscape add the spaces (it should not).
4. Save the document
5. Reload the document (File>Revert).
-> Now the text element shows one space only (as expected).