Potential fix for MLA style; question about `source` variable

We are generating citeproc-json that doesn't include the `source` or `access` fields.  Right now, for the MLA style, this is causing the medium of publication to be dropped, but it probably should be "Web".

A general question is:  should we be giving these values?   According to [the spec](http://citationstyles.org/downloads/specification.html#standard-variables), `source` is "from whence the item originates (e.g. a library catalog or database)".  But, we're generating these styles from data in our (PMC) archive, so it's not at all clear.  Should we use `PMC`?  Then, that appears in the citation, but it doesn't really seem to make sense.