Robots.txt plugin does not add slash after excluded facets path

The Robots.txt plugin automatically excludes sitemap items that show faceted navigation. However, these excluded URLs are not suffixed with a slash. As a result, all sitemap items whose name merely start with the same part will also be excluded, since crawlers use simple substring matching for the disallowed URLs [1].

Steps to reproduce:
1. add the Robots.txt plugin to a archetype project
2. create a root-level sitemap item called "products" whose relativecontentpath property points to a faceted navigation node
3. request http://localhost:8080/site/robots.txt

Expected: the output is:

User-agent: *
Disallow: /site/products/

Actual: the output is:

User-agent: *
Disallow: /site/products

(note the missing trailing slash).

URLs like "/products-archive" and "/productstorage" would now also be disallowed since they also start with "/products".

[1] http://en.wikipedia.org/wiki/Robots_exclusion_standard#Disadvantages