Bugfix: Encoding of country code in FormatOutOfCountryCallingNumber

[jojomi]

Minimal testcase:

phonenumber, err := libphonenumber.Parse("+3587884512974", "DE")
fmt.Println(libphonenumber.FormatOutOfCountryCallingNumber(phonenumber,
"DE"))

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This change is 

[ttacon]

Hi! Thanks for the PR! A few questions:

1. Can you add a test to the test code to show what you're trying to fix here as well (I know that it's in the comment above but if it's a test case we're fixing it should also go into the unit tests)?

2. What exactly is the issue that you're trying to resolve? I'd prefer not to incur multiple allocations to do an integer -> []byte conversion.

Thanks!

[jojomi]

Thank you for having a look.

1. I can try, but currently my time is limited. I am okay with leaving this PR open until added
2. Is there a quicker way to get from an int to []byte? The point is that the bit wizardry seems wrong there, the country code integer just needs to be printed. The current code yields seemingly random (non-numerical) characters depending on what character codes are mapped after shifting around.