Add a non-breaking space to _s_post_nav().

Sometimes the title length of a previous or next post would cause
the arrow (larr/rarr) in the link to be wrapped to the next line.
Adding a non-breaking space between the title and the arrow will stick
it to the last word in the title.
Merge pull request #527 from kovshenin/master

Add a non-breaking space to _s_post_nav().