Fix generic client interceptor resolution

[murgatroid99]

This moves all of the interceptor resolution logic to the call time to avoid needing to do method name lookups. This should fix #363. I also ended up deleting a couple of helper functions in client_interceptors.js that were only used for the previous iteration of this functionality. We could probably also remove Client.prototype.$method_names, but it's used in one of the tests and I didn't feel like trying to figure out how to change it.

[nicolasnoble]

Something tells me that the original intend of the code was to basically be _.isArrayLike, by checking if it had the length property.

[nicolasnoble]

Also, unrelated: shouldn't we be doing something like:

if (!_.isArrayLike(options.interceptor_providers) && _.isSet(options.interceptor_providers)) {
  throw new client_interceptors.InterceptorConfigurationError('interceptor_providers needs to behave like an array')
}

[murgatroid99]

The old code wasn't checking if they were array-like, it was checking if they were non-empty arrays. Note that that line appeared after the line that conditionally set those variables to empty arrays. Strictly speaking, it's not exactly the same behavior now, but I suspect that the old behavior of accepting two empty arrays was more error-prone.

And for now, I don't think it makes a big difference to do that additional check. There's a rabbit hole there of checking if each element is a function, and checking if each element of interceptors is actually an interceptor. I just want to get this working, especially for people who aren't using interceptors at all.