fix(Image): use instance check on isImage

[twlite]

This PR updates static Image.isImage method to initially verify the object via instance check.

Not sure if the verification should be done via toString though, because of the following issue:

// here we have a class named Demo
class Demo {
    // We return "Demo" here
    get [Symbol.toStringTag]() {
        return "Demo";
    }
    // this is implemented in same way as Image
    static isDemo(obj) {
        return Object.prototype.toString.call(obj) === "[object Demo]";
    }
}

// here we have another class named Demo2
class Demo2 {
    // Here we are returning the same "Demo" string
    get [Symbol.toStringTag]() {
        return "Demo";
    }
    // this too is equivalent to Image's isImage method
    static isDemo(obj) {
        return Object.prototype.toString.call(obj) === "[object Demo]";
    }
}

// instantiating each of these classes
const demo1 = new Demo();
const demo2 = new Demo2();

// since we are just verifying via toString return value, we don't know if demo1 and demo2 are actually equal
console.log(`${demo1.toString()} ${demo2.toString()}`) // Demo Demo
console.log(`${Demo.isDemo(demo2)} ${Demo.isDemo(demo1)}`) // true true

Due to this, Image.isImage may not always validate the actual Image which can cause problems. Let me know if toString implementation should not be removed.

[codecov[bot]]

Codecov Report

Base: 84.43% // Head: 84.42% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (acc160d) compared to base (e3ba8d4). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           legacy     #611      +/-   ##
==========================================
- Coverage   84.43%   84.42%   -0.01%     
==========================================
  Files         163      163              
  Lines        5697     5696       -1     
  Branches     1424     1424              
==========================================
- Hits         4810     4809       -1     
  Misses        790      790              
  Partials       97       97              

Impacted Files	Coverage Δ
src/image/Image.js	94.25% <100.00%> (-0.07%)	:arrow_down:

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[targos]

Does this fix a real issue?

[twlite]

Does this fix a real issue?

With instance check, it should only verify the instances of Image. Basically similar approach to the current one but a bit more strict.

[targos]

isImage was implemented in the current way to allow for interoperability with multiple versions of image-js. We are not going to do the same with the future TypeScript version of the project but I don't want to risk breaking existing code in the legacy branch.

[twlite]

isImage was implemented in the current way to allow for interoperability with multiple versions of image-js. We are not going to do the same with the future TypeScript version of the project but I don't want to risk breaking existing code in the legacy branch.

alright thanks