Fix encoding issue, lower the limit for multipart requests

[CristianCantoro]

As per subject.
Tested with ~ 5kB request was return an error (request too large)

[CristianCantoro]

Rewrote history, removed the else: pass part.

[valhallasw]

I still think it would be better to have

if isinstance(v, str):
    v = v.decode('utf-8')

...

(k, v.encode('utf-8'), ...)

as we tell the server the data will be utf-8. I think it's cleaner to actually check this, instead of just passing the str (which could also contain, say, latin-1 data).