Fix multipleOf constraint for integers

[mittsh]

Proposal to fix the multipleOf constraint for integers. In fact, it wasn't working so far for the type integer. If the value passed was a Swift Int, we got value as? Double being evaluated to nil.

I made some changes, to try to coherce both number and value into an Int to use the standard modulo operator. And fallback on floating point comparison.

Also, I've replaced the previous flooring method by truncatingRemainder(dividingBy:) for floating point division.

Let me know what you think @kylef I'm happy to make any changes if coding style or implementation isn't matching the current project standards. Thanks in advance!

[kylef]

Looks like these changes are breaking one of the tests (https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/8f867168d17497a775141704960906db83634a04/tests/draft4/multipleOf.json#L48-L52)

JSONSchemaTests.JSONSchemaCases
  testEverything, XCTAssertEqual failed: ("false") is not equal to ("true") - Failed validation: ["0.0075 is not a multiple of 0.0001"]
  /Users/travis/build/kylef/JSONSchema.swift/Tests/JSONSchemaCases.swift:113

      case .invalid(let errors):
        XCTAssertEqual(result.valid, test.value, "Failed validation: \(errors)")
      }

Could you take a look into this please?

[mittsh]

Absolutely, yes. I'll have a look. Thank you! 😄