isFunction is set incorrectly inside context._get

[prashn64]

isFunction is meant to be true only on non dust functions. isFunction is set on all functions inside this _get, but a dust body function could be returned by _get.

One issue this causes is calling a dust body function with no parameters in dust.helpers.tap, when dust functions expect a chunk and a context.

[kate2753]

Implementation of dust.helpers.tap was changed not to rely on isFunction flag. Are there any other cases we need to worry about? Can we remove this flag from core?

Based on the way isFunction flag is currently implemented, we can use typeof foo === 'function' instead.

[prashn64]

isFunction isn't used anywhere else in the core or helpers. You could argue that someone may be relying on that flag, but I highly doubt it since it's undocumented, and has an ambiguous name. If anyone feels strongly about it, we can roll it out as a deprecation and then remove it, but I'm all for just removing it.