Do not log a warning when observing an already observed object key path

[siyusong]

As mentioned in the documentation, observing an already observed object key path using - observe:keyPaths:... methods should "result in no operation". However, the current implementation logs a warning/info in that case. Since "observing an already observed object key path" is not a misuse of the API, I think the warning is unnecessary. I am relying on the aforementioned behavior in one of my project and found the NSLog messages a little bit annoying.

This is also the only occurrence of NSLog in the whole project.

[nlutsenko]

Do you think we should do something like NSAssert does where the statement is logged only when debugging, but never in release? I am ok merging this as is, to be honest, but it might be helpful to eliminate unwanted behavior...

[nlutsenko]

For example:

#ifdef NS_BLOCK_ASSERTIONS
NSLog(...);
#endif

[siyusong]

Just my two cents, but I don't think it's very easy to tell whether the user is unintentionally observing the same object multiple times ("misusing" the API), or if this is actually what they want. From a pure API contract perspective, it is not a misuse of the API since the documentation explicitly allows it, and we should not "penalize" the user who is using the API in a way that is allowed.

[nlutsenko]

Yup, this is valid concern. And taking into account that we have documentation around this method that we are allowing double-observation and it's a no-op at that point - I fully agree with the change. Merging now.