fix: Properly set the WebView config when it is provided

[dpogue]

Platforms affected

iOS

Motivation and Context

Fixes GH-1423 (based on the comment on https://github.com/apache/cordova-ios/commit/4c8136345ae9622646ee7e4eb60ba2b37ad44fbe#r142043240)

Description

Check that the engine class instance responds to the initWithFrame:configuration: selector rather than the engine class itself (because it's not a static class method).

Testing

All unit tests pass, but I don't think we have test coverage for this particular feature.

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)

[dpogue]

/fyi @msmtamburro @Durzan666

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.35%. Comparing base (2240ebc) to head (93a95e6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1436   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files          16       16           
  Lines        1825     1825           
=======================================
  Hits         1430     1430           
  Misses        395      395           

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