The API request add_field method has had a deprecation warning for a

while about passing in values that are not either str or bytes. This

change replaces the warning+cast with an exception.
There were a few places inside RBTools itself that were still hitting

this, all in the post implementation. These have been fixed up to format

the values as strings.

Ran unit tests.
Audited call sites to verify that we were passing string types

  everywhere.
Posted changes, including ones that needed to upload binary files.