Simplify return logic in handleActions; remove typeof check

[yangmillstheory]

http://stackoverflow.com/a/4725697/2419669

For undeclared variables, typeof foo will return the string literal "undefined", whereas the identity check foo === undefined would trigger the error "foo is not defined".

For local variables (which you know are declared somewhere), no such error would occur, hence the identity check.

defaultState is declared as a parameter, so the typeof check is unnecessary (like here).