Skip unset attributes

[cydanil]

Hi Micah,
Thanks again for your library!

This PR aims to solve a parsing issue when time are not set.

Drinks at allrecipes.com tend to not have a cooking time (cookTime) set.
In their schema, the value is set to null:

{
  "@context": "http://schema.org",
  "@type": "Recipe",

[...]

  "prepTime": "P0DT0H10M",
  "cookTime": null,
  "totalTime":  "P0DT0H10M",
}

In their webpage, the null value is used to hide fields. A couple of examples are:

https://www.allrecipes.com/recipe/237874/simple-moscow-mule/
https://www.allrecipes.com/recipe/272987/watermelon-mojitos/

Here, we add a check for None and remove the key from the recipe dictionary.
I've also added a unit test with one of the above recipe checking that the key is correctly removed.

Thanks!
Cyril

[cydanil]

I added the attribution for the test recipe I added, but I'm not sure what's the license...

[micahcochran]

Hello Cyril,

Thank you for the patch. Everything looks good.

I'd say the license for Allrecipes is Proprietary. I've consciously shied away from proprietary sources, but this seems to be fair use.

Travis CI isn't working, probably due to their migration from .org to .com website and I have probably not updated some setting. :-(

Thanks,
Micah