WP-5202 Switch mock-aware requests to real requests before finalizing them

[evanweible-wf]

Problem

When MockTransports are installed with fallThrough enabled, we check to see if each request is mock-aware and if it would be handled by an expectation or a mock handler. If it wouldn't be, we switch it to a real request so that it doesn't get stuck (i.e. "fall-through").

Currently, we finalize the request prior to making this check. It turns out that this can have an adverse effect on MultipartRequests due to their divergent browser and mock implementations. In the browser, the MultipartRequest files map accepts dart:html.File and dart:html.Blob instances, but those types cannot be imported in any platform other than the browser. As a result, attempting to finalize the mock-aware MultipartRequest will fail if either a File or Blob instance is attached because it does not recognize them.

Solution

The solution is to switch to a real request (if applicable) prior to finalizing the request, which will avoid the issue altogether if the request isn't destined to be handled by a mock handler or expectation anyway.

Testing

• CI passes (applicable test has been updated – you can verify by checking out 76e6488 and running the tests without the fix)

Code Review

@Workiva/web-platform-pp

[aviary-wf]

Raven

Number of Findings: 0

[codecov-io]

Codecov Report

Merging #290 into master will not change coverage.
The diff coverage is 100%.

[jayudey-wf]

QA +1

• Testing instruction
• Dev +1's
• Dev/QA +10 with detail of what was tested
  • verified that test was appropriately failing without the corresponding source code update
• Security review (if required)
• Unit tests created/updated
• All unit tests pass
• Rosie has run and reports properly the release the ticket will be included in

Merging into master.