Support querying numeric HTML properties from elements::Element::prop()

[johnDeSilencio]

Preface

Hi, Jon! Longtime fan of yours. Your Crust of Rust series was HUGE for me going from a beginner Rust developer to more intermediate (I've got a long way to go before the rank of "master" 😄). Thank you for all the amazing work you do and how you give back to the community!

Problem

I'm building my own personal portfolio website. One of the features of the application is a terminal emulator, letting you run fake commands on the website.

One of the commands is clear, which clears the screen and the history of all previous commands. To write a functional test for this feature, I'd like to be able to tell that my <div id="previous-commands> has no children which I check through the childElementCount property.

However, when I try use fantoccini::elements::Element::prop("childElementCount"), I get the following error:

...webdriver returned non-conforming response: Number(0)

Proposed Solution

It appears that the fantoccini::elements::Element::prop method currently only handles Strings, Bools, and Null values from serde_json and returns an error for everything else. This PR also supports matching serde_json::Number to support getting numeric properties like childElementCount.

Tests

I've also added another assert_eq!() to the element_test function. Let me know if you'd like me to add more test coverage or to find another way to support numeric HTML properties.

Functional Tests

I used this fork in the E2E tests for my website and now my functional tests pass:

[jonhoo]

Thank you^2!

Arguably, prop should just return a serde_json::Value instead of coercing everything to a string. I'm honestly not quite sure why I made it be string only 🤔 I think it's because attribute can only return strings, and I guess I incorrectly assumed that the same would apply to prop. But property values can be "any ECMAScript language type". Given we're going to have to cut a breaking change for #285 anyway, let's maybe just go ahead and change prop to return serde_json::Value right now?

[johnDeSilencio]

Hey, Jon!

I've made the change you suggested so that prop() now returns a serde_json::Value. I also added a doc comment example to the prop() method, but feel free to let me know if you think I should take it out or change it