Issue Description:

ISO8601DateFormatter shows unexpected and against-its-documentation behavior related to numeric time zone offset (ref. RFC 3339).

From the ISO8601DateFormatter.Options Documentation:

The Option withInternetDateTime is composed and contains {{withColonSeparatorInTimeZone }}amongst other things. The expected behavior for formatting a date into a String is reflected in the Example Column above.

Actual Behavior:

Figure 1 - ISO8601DateFormatter

let input = "2017-01-06T08:45:00+00:00"
let formatter = ISO8601DateFormatter()
formatter.formatOptions = .withInternetDateTime
formatter.timeZone = TimeZone(identifier: "GMT")!
formatter.string(from: formatter.date(from: input)!)
// prints "2017-01-06T08:45:00Z"

Expected "2017-01-06T08:45:00+00:00"  

Even though Z stands for +00:00, this is not according to the documentation and/or the specified {{ISO8601DateFormatter.Options }}value.

Same applies for Figure 2, the underlying DateFormatter

Figure 2 - DateFormatter

let input = "2017-01-06T08:45:00+00:00"
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
formatter.timeZone = TimeZone(identifier: "GMT")!
formatter.string(from: formatter.date(from: input)!)
// prints "2017-01-06T08:45:00Z"

 Expected "2017-01-06T08:45:00+00:00"    

ZZZZZ is expected to output the "long" numeric time zone offset. Showing Z is inconsistent with actual offset behavior.

To show even more inconsistency, Figure 3 would expect the same behavior as above, but results in this:

Figure 3 - DateFormatter

let input = "2017-01-06T08:45:00+00:00"
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
formatter.timeZone = TimeZone(identifier: "GMT")!
formatter.string(from: formatter.date(from: input)!)
// prints "2017-01-06T08:45:00+0000"

Expectation, based on Fig. 1 & Fig. 2: "2017-01-06T08:45:00Z"      

This only applies for zero offsets (where Z would be appropriate), but because this is showing inconsistent behavior in documentation and {{ISO8601DateFormatter }}Implementation, I decided to open this bug.

If requested, I can look through the source code and open a pull request, if I manage to find the underlying issue here.