Fix issues with unit-tests.

[bummzack]

AllowedCountries are now JSON encoded.
Fix changes that were made to the omnipay Dummy gateway.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0bd2bdd). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #660   +/-   ##
=========================================
  Coverage          ?   46.72%           
  Complexity        ?     1559           
=========================================
  Files             ?      115           
  Lines             ?     4546           
  Branches          ?        0           
=========================================
  Hits              ?     2124           
  Misses            ?     2422           
  Partials          ?        0

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0bd2bdd...3fd53f2. Read the comment docs.