Only pass bytes to urllib3.
Only pass bytes to urllib3.

This should resolve requests-oauthlib's problems with uploading binary
data, as demonstrated in kennethreitz/requests#1252.

gras100/asks-oauthlib,jsfan/requests-oauthlib,dasevilla/requests-oauthlib,singingwolfboy/requests-oauthlib,dongguangming/requests-oauthlib,jayvdb/requests-oauthlib,lucidbard/requests-oauthlib,abhi931375/requests-oauthlib,requests/requests-oauthlib,elafarge/requests-oauthlib,jayvdb/requests-oauthlib,dasevilla/requests-oauthlib,sigmavirus24/requests-oauthlib