Fix Lazy Loading

[geikha]

Fixes #290

This was SO hard to catch...

Basically lazy loading wouldn't work properly because the custom readWithInfo method added to Buffer wasn't explicitly returning the buffer itself when onlyHeader (which is set according to doNotReadYet) was set to true. It was returning nil, which would result on the buffer not being added to the buffers IdentityDictionary.

readWithInfo { |startFrame = 0, argNumFrames = -1, onlyHeader = false, onComplete|
		...
		^if(failed) {
			...
		} {
			if(onlyHeader.not) {
				if(argNumFrames > 0) { numFrames = argNumFrames };
				this.allocRead(path, startFrame, numFrames, completionMessage: { |b|
					onComplete.value(b)
				});
			} { this; } // this wasn't here before
		}
	}

Tested on my Windows 10 machine.