DEVX-6362 fix auto advance for Numbers API

[superchilled]

Reason for this PR

To fix a bug when setting the auto_advance parameter to true when making requests with the Numbers class list and search methods.

The cause of this bug was that auto advance relied on the Namespace#iterable_request, the implementation of which didn't work for the Numbers API because responses from that API don't include a record_index value.

What this PR does

• Implements a specific version of iterable_request in the Numbers class which over-rides the version in Namespace.

[codecov-commenter]

Codecov Report

Merging #236 (a8a8995) into dev (e4e4f79) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #236      +/-   ##
==========================================
+ Coverage   97.69%   97.71%   +0.02%     
==========================================
  Files          70       70              
  Lines        1388     1401      +13     
==========================================
+ Hits         1356     1369      +13     
  Misses         32       32              

Impacted Files	Coverage Δ
lib/vonage/namespace.rb	100.00% <ø> (ø)
lib/vonage/numbers.rb	100.00% <100.00%> (ø)

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