Fixes to meta 'published' and 'updated' methods, on ruby 2.0.0

**This fix assumes you're using the published plugin**

The issue i'm describing may be a ruby 2.0.0 thing (as this is what i'm using pith with most of the time)
I've found that if the published and updated files aren't wrapped as strings, YAML may parse the published and updated dates prior to assignment to the page meta data. 

This has tripped me up a few times, and may have tripped up others as well

## How to reproduce
Create a page where published date is not wrapped in strings.
This will raise an 'TypeError: no implicit conversion of Date into String'

    ---
    published 2013-09-19
    ...

    <h1>Hello world</h1>


Wrapping the date as a string will work

    ---
    published '2013-09-19'
    ...

    <h1>Hello world</h1>