Use "aws_access_key_id" and "aws_secret_access_key" for reads, not just unload

[JoshRosen]

This is an updated version of #32 with two additional changes:

• Do not mutate the SparkContext's hadoopConfiguration.
• Refactor tests so that they're capable of catching this bug.

[codecov-io]

Current coverage is 87.93%

Merging #55 into master will increase coverage by +0.67% as of 229930d

@@            master     #55   diff @@
======================================
  Files           10      10       
  Stmts          369     373     +4
  Branches        87      85     -2
  Methods          0       0       
======================================
+ Hit            322     328     +6
  Partial          0       0       
+ Missed          47      45     -2

Review entire Coverage Diff as of 229930d

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[JoshRosen]

Going to merge this now, with the commit authorship credited to @koeninger.