Doesn't keep Symbol properties

[stnever]

Hello! When omitEmpty is called with an object that has Symbols as property keys, those keys are missing from the result:

const omitEmpty = require('omit-empty')
const assert = require('assert')

let gte = Symbol('gte'),
  lt = Symbol.for('lt')

test('should clean some symbols', () => {
  let actual = omitEmpty({
    userId: 1,
    age: {
      [gte]: null,
      [lt]: 20
    }
  })

  let expected = {
    userId: 1, 
    age: {
      [lt]: 2
    }
  }

  // fails: the returned object is just { userId: 1 }
  assert.deepEqual(actual, expected)
})

This happens because Object.keys() only sees string keys, in this line: https://github.com/jonschlinkert/omit-empty/blob/69954742d6a8f5bdf3056c5f9fc6ad355595a6ae/index.js#L15

This could be fixed by using Object.getOwnPropertySymbols() in addition to Object.keys().

(The reason I'm using symbols as property keys is because of Sequelize's recommended query practices).

[stnever]

I'm just reporting this issue; I've already worked around it.

I'm hesitant to provide a PR because I'm unsure if getOwnPropertySymbols() might cause errors in old environments, or if it would be a breaking change.