Add a BC layer in PhoneNumber serialize method

[odolbeau]

While trying to upgrade the PhoneNumberBundle (see odolbeau/phone-number-bundle#193) my application wasn't able to deserialize an "old" serialized phone number.

I added a BC layer in PhoneNumber to allow deserialisation of an "old" CountryCodeSource (from int to enum).

[giggsey]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.99%. Comparing base (b01b0b6) to head (6cc74fd).
Report is 16 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #705      +/-   ##
============================================
+ Coverage     85.98%   85.99%   +0.01%     
- Complexity     1302     1303       +1     
============================================
  Files            31       31              
  Lines          3481     3484       +3     
============================================
+ Hits           2993     2996       +3     
  Misses          488      488              

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[giggsey]

I expect Google will do a new release later this week