Incorrect check if body is a string

[udalrich]

The code checks if body is a string by doing if body.lower:. That passes if it is a string, but throws an exception if body is a io.BufferedReader, such as what one gets from

with open('someFile.bin', 'rb') as f:
  with boddle(body=f):
     pass

It would also fail if the file-like object happens to have a lower method/attribute

[keredson]

i think adding support for passing file-like objects in for body is a good idea. open to a PR. :)