OISRateHelper should take a QuoteHandle for overnightSpread parameter

[trentmaetzold]

The constructor for OISRateHelper takes a float for the overnightSpread argument. It would be nice if it took a Quote object so that the curve doesn't need to be rebuilt if spreads widen/tighten. (It's probably not a huge impact day to day, but would be nice if it functioned the same as a spread-less curve i.e. when quotes update, the curve gets updated)

I haven't done much testing here, but seems like it might be needed for some curves to specify conventions for the spread as well (might be more in Ibors).

[trentmaetzold]

I'm seeing that the SwapRateHelper takes a Quote for the spread so Ibors are covered. It's probably just the Fed Funds curve that uses a spread in OIS, but may be nice to have that function as well there. I'll try to take a look at it when I get more time.

[lballabio]

Ok, thanks!