rss_converter_twitter.com.xsl: preserve spaces in tweet content

Wrap the tweet content into a span element with a CSS style attribute
set to "white-space: pre-wrap", this allows to have the spaces rendered
like on the twitter web page: with spaces and newlines preserved.

This is especially desirable if the tweet content contains any ASCII
art, like in https://twitter.com/sarahjeong/status/955651919279722496