Added check for existing key, if key already exists, the entry is cha…

[GregersBoye]

…nged to array. This way it supports multiple entries of the same key - namely image.

[AramZS]

Excellent work @GregersBoye - I've made some slight modifications to assure that users accessing the system the old way will retain access. By switching it to an array it would have broken any current users systems who were expecting the old method of response. Instead I've set it up to add the additional values into an array under a different key. You can see my alterations above. Since the current maintainer seems to have checked out, I'm maintaining this branch forward of this repo at https://github.com/AramZS/opengraph

Thanks!