hasAttributes filter method supports attributes with omitted boolean values

[artemruts]

This solves an issue when JSXElement's hasAttribute filter method can't find attributes where boolean value (true) was omitted.

E.g., given this jsx:

<FooBar baz />

If I try to do something like this:

const jsx = Collection.fromNodes(/* jsx nodes */)
  .findJSXElements()
  .filter(
    JSXElementCollection.filters.hasAttributes({
      baz: v => v === null
    })
  );

JSXElement will throw an error TypeError: Cannot read property 'expression' of null since there is no value.

jscodeshift/src/collections/JSXElement.js

Line 100 in 595b15c

const actual = Literal.check(value) ? value.value : value.expression;

balylon7 AST for above jsx code looks like this:

{
  "type": "JSXAttribute",
  "start": 8,
  "end": 11,
  "loc": {
   /* ... */
  },
  "name": {
    "type": "JSXIdentifier",
     /* ... */
    },
    "name": "baz"
  },
  "value": null
}

---

I also ran npm run docs since I modified a few files, this produced lots of doc updates 😨
Main changes are in JSXElement.js and JSXElement-test.js files.

[artemruts]

@fkling Felix, I know you're busy but can I please get any feedback on this or is there someone who can review this PR?

Thanks.