Use WordPress HTTP class for SimplePie requests

[DD32]

Whilst SimplePie's request class may work in most cases, There will be times when it wont. The HTTP class also has more functionality than the SimplePie class.

AFAIK, SimplePie will expand it in the future(rmccue) however, Its right now i'm concerned about.

The HTTP Class also allows people to disable transports which do not work for them, Or to set proxies/blacklistings, For that reason alone, SimplePie should utilise the HTTP Class.

Attached patch converts it, I've done small ammount of testing, There doesnt appear to be any problems, However, Testing will be needed to ensure its acting 100% like the SimplePie_File, Mainly with regards to headers.

(In [10687]) Use WordPress HTTP class for SimplePie requests. Props DD32. fixes #9247