Layout properties of the document must be integers

If you set layout properties through the added UI on 7.x-2.x branch and export documents, the layout properties are exported as JSON strings. Apple News Format document requires that they be numbers. If you preview a document in News Preview, you get the following errors and the document fails to load:

Error: "columns" was <"7"> but should be (null or null (JSON) or number)
layout -> columns

Error: "width" was <"1024"> but should be (null or null (JSON) or number)
layout -> width

Error: "margin" was <"70"> but should be (null or null (JSON) or number)
layout -> margin

Error: "gutter" was <"40"> but should be (null or null (JSON) or number)
layout -> gutter

Error: No useful layout found.
layout