fix: closeTo should check value's type before assertion

`Assertion#closeTo` passes tests that should be failed.

e.g.
```javascript
expect([1.5]).to.be.closeTo(1.0, 0.5); // PASS???
expect(1.5).to.be.closeTo("1.0", 0.5); // PASS???
```

so, I fixed it.
```javascript
expect([1.5]).to.be.closeTo(1.0, 0.5); // throw "expected [ 1.5 ] to be a number"
expect(1.5).to.be.closeTo("1.0", 0.5); // throw "expected '1.0' to be a number"
```