explicitly specify httr::POST()'s value for encode

[wibeasley]

related #429 & to @brandonpotvin's implicit suggestion, REDCapR should be tighter with httr::POST()'s encode parameter. Right now, it uses httr's default of "multipart". Looking at some things closely (like the API playground & httr documentation), it looks like most (but not all) should be "form".

This hasn't made a difference with any scenarios that I'm aware of. But I'd like to standardize now, since some institutions are starting unconventional approaches. I feel like following the API playground keeps more people centered.

[wibeasley]

@brandonpotvin, this shouldn't be noticeable to you. But if you notice something weird/breaking in future cases, let's revisit this issue first.

[brandonpotvin]

@wibeasley, everything seems to be working as expected. I will let you know if I find something. Thanks.