Fix String type with byte format

[mattia]

When the spec defines a string property with a byte format at the moment a String property is generated, but a Data property should be generated instead.

With input:

test_property:
  type: string
  format: byte

Before:

var testProperty: String

After:

var testProperty: Data

[kean]

Thanks for making the change! I appreciate it.

[mattia]

Thank you for the library!