[Swift] Very large numbers are not converted to string correctly

[joeboyscout04]

Description

When working with very large numbers (in this case, timestamps in nanoseconds), the default printing by swift results in an exponent character in the string. For example:

let largeNumber: Double = 1.52639622817888e+16
print("\(largeNumber)") // results in "1.52639622817888e+16"

Swagger-codegen version

2.3.1

Steps to reproduce

This is reproducible in an Swift project that uses Doubles for request parameters, as long as the input Double is sufficiently large.

Related issues/PRs

TBD

Suggest a fix/enhancement

Proposed fix is to switch to using NumberFormatter()

let formatter = NumberFormatter()
let num = NSNumber(value: bigNumber)

print(formatter.string(from: num)) // results in "15263962281788800"