Do not left strip content when after having parsed metadata

When parsing items and separating content from metadata, Nanoc strips the content: https://github.com/nanoc/nanoc/blob/3.7.0/lib/nanoc/data_sources/filesystem.rb#L271

This is bogus and prevents the following Markdown from being correctly compiled:

    ----
    title: I start with a code block
    ----
        this is a code block
        this is a code block

Calling `#strip` removes the leading spaces and breaks the code block.