Fix default in getAuthenticationUrl to pass if requested

[rogebrd]

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• SDK Code Change
• Example/Test Code Change

Validation

• Does tox pass?
• Do the tests pass?

[codecov]

Codecov Report

Merging #271 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   46.91%   46.91%   -0.01%     
==========================================
  Files          31       31              
  Lines       77989    77988       -1     
  Branches     6445     6444       -1     
==========================================
- Hits        36590    36589       -1     
  Misses      41348    41348              
  Partials       51       51              

Flag	Coverage Δ
#unit	46.91% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
dropbox/oauth.py	55.04% <100.00%> (-0.21%)	⬇️

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