fix(HttpApi): error when defining $default path in OpenApi

[keetonian]

Issue #, if available:
N/A

Description of changes:
Fixes how SAM treats the default path (allows for a string uri)

Description of how you validated changes:
Tested and deployed template.

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[alexw91]

Codecov Report

Merging #1498 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1498   +/-   ##
========================================
  Coverage    94.13%   94.13%           
========================================
  Files           78       78           
  Lines         4925     4928    +3     
  Branches       990      991    +1     
========================================
+ Hits          4636     4639    +3     
  Misses         134      134           
  Partials       155      155           

Impacted Files	Coverage Δ
samtranslator/open_api/open_api.py	91.48% <100.00%> (+0.11%)	⬆️

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[codecov-io]

Codecov Report

Merging #1498 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1498      +/-   ##
===========================================
+ Coverage    94.13%   94.13%   +<.01%     
===========================================
  Files           78       78              
  Lines         4925     4927       +2     
  Branches       990      991       +1     
===========================================
+ Hits          4636     4638       +2     
  Misses         134      134              
  Partials       155      155

Impacted Files	Coverage Δ
samtranslator/open_api/open_api.py	91.45% <100%> (+0.07%)	⬆️

---

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 aba498f...5c30ddb. Read the comment docs.