Fix NameError in delete_paste.

This fixes problem:
NameError: global name 'request_string' is not defined

when you call:
PastebinAPI().delete_paste(api_dev_key, api_user_key, api_paste_key)
Merge pull request #2 from ivanalejandro0/bug/request_string-is-not-defined

Fix NameError in delete_paste.