Fix HTTPBodyStream warning.
Fix HTTPBodyStream warning.

The "potentially fatal bug" warning only applies to iOS version 6 and
lower, so let's only warn in that case. Also, we can prioritize
SPDYBodyStream over HTTPBodyStream, to allow the app to set both for
when it doesn't know whether SPDY will get used or not.

johnkdoe/CocoaSPDY,kgoodier/CocoaSPDY,johnkdoe/CocoaSPDY,kgoodier/CocoaSPDY