Fix Utils.joinURLs on Windows

[JoshRosen]

This PR fixes an issue where spark-redshift would not work properly on Windows because the Utils.joinUrls method implicitly used the platform-specific path separator via File.

[codecov-io]

Current coverage is 94.81%

Merging #93 into master will decrease coverage by -0.03% as of dfc171c

@@            master     #93   diff @@
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  Files           11      11       
  Stmts          446     444     -2
  Branches       109     109       
  Methods          0       0       
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- Hit            423     421     -2
  Partial          0       0       
  Missed          23      23       

Review entire Coverage Diff as of dfc171c

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[JoshRosen]

Going to merge this now.