OFHTTPClient: Add support for Basic Authorization
OFHTTPClient: Defaults for Content-{Type,Length}

In case the entity body has been set, but the headers don't contain
Content-Type and/or Content-Length, assume a default of
"application/x-www-form-urlencoded; charset=UTF-8" for the Content-Type
and set the Content-Length to the number of bytes of the entity body.
OFHTTPRequest: Add -[setEntityFromString:]
OFString+URLEncoding.m: Better RFC 1738 compliance
OFString+*.m: Add a few missing autorelease pools

Those were only a problem if the user created its own OFString subclass,
as calls to -[UTF8String] could then create a new, autoreleased C
string. As OFString_UTF8 simply returns the internal C string, this
hasn't been a problem when not subclassing OFString.