fix sep detection for single column quoted file

[ben-schwen]

Closes #7366

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.13%. Comparing base (1685a3b) to head (048fabb).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7370   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files          85       85           
  Lines       16610    16618    +8     
=======================================
+ Hits        16466    16474    +8     
  Misses        144      144           

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[github-actions]

• HEAD=fread_quote_sep slower P<0.001 for memrecycle regression fixed in #5463
• HEAD=fread_quote_sep slower P<0.001 for isoweek improved in #7144
  

Generated via commit 048fabb

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 1 seconds
Installing different package versions	44 seconds
Running and plotting the test cases	2 minutes and 39 seconds

[aitap]

This feels like a relatively complicated special case, but I wasn't able to simplify it.