Skip modifying argument if SpreadElement appears immediately beneath assert

[twada]

Skip modifying argument if SpreadElement appears immediately beneath assert.

repro case:

const args = [true === false, 'message'];
assert(...args);

assert(...args) looks like one argument syntactically, however there are two arguments actually.

babel-plugin-espower works at the syntax level so it cannot handle SpreadElement that appears immediately beneath assert.

fixes #28
refs avajs/ava#2004