Decode ResRobot response using ISO-8859-1.

The API documentation notes that "the header of the response may say it
ISO-8859-1", which it does, and that "This API uses UTF-8 as encoding
for both parameters and responses... Please force UTF-8."

However, the response I get doesn't decode properly as UTF-8: Swedish ö,
for instance, is represented by the byte 0xF6, matching ISO-8859-1,
instead of the proper UTF-8 representation 0xC3B6.