code span doesn't preserve space

[weakish]

Consider this code span: `grep '    '`.  All implementations listed on
[babelmark](http://babelmark.bobtfish.net) just simply wrap it as:

    <code>grep '    '</code>

Viewed in browsers, the above code is equivalent to:

    <code>grep ' '</code>

I think spaces should be preserved in code span,which maintains consistence
with code block.  For example, the following HTML code:

    <code>grep '&nbsp;&nbsp;&nbsp; '</code>