Diagnostics

Hi,

There is a small issue where a diagnostic message  is shown when the attribute elements of xsl:preserve-space or xsl:strip-space ends or starts with a space. The message (which contains a typo) is:

> XSLT: Exected list of valid names (with declared prefixes) in 'elements' but found ''

```xslt
<xsl:strip-space elements="Artikel Paragraaf Noot "/>
```