page.mouse.click fails when x/y are strings

[Jarrod Overson]

Actual:

Passing numbers as strings to page.mouse.click(x,y) causes it to fail, e.g.

await page.mouse.click('100', '100');

gives error:

UnhandledPromiseRejectionWarning: Error: Protocol error (Input.dispatchMouseEvent): Invalid parameters x: double value expected; y: double value expected

Expected:

1. Method should throw on invalid type.