Add support for overriding request options.

[jmdobry]

• npm test succeeds
• Pull request has been squashed into 1 commit
• I did NOT manually make changes to anything in apis/
• Code coverage does not decrease (if any source code was changed)
• Appropriate JSDoc comments were updated in source code (if applicable)
• Appropriate changes to readme/docs are included in PR

Fixes #501
Fixes #618
Fixes #621

Before

drive.files.export({
  fileId: 'asxKJod9s79', // A Google Doc
  mimeType: 'application/pdf',
  encoding: null // This is ignored
}, , function (err, buffer) {
  // Oh no! buffer got incorrectly interpreted as a string!
});

After

drive.files.export({
  fileId: 'asxKJod9s79', // A Google Doc
  mimeType: 'application/pdf'
}, {
  encoding: null // Make sure we get the binary data
}, function (err, buffer) {
  // Nice, the buffer is actually a Buffer!
});

[codecov-io]

Current coverage is 88.56% (diff: 100%)

Merging #623 into master will not change coverage

@@             master       #623   diff @@
==========================================
  Files             8          8          
  Lines           271        271          
  Methods          41         41          
  Messages          0          0          
  Branches         59         59          
==========================================
  Hits            240        240          
  Misses           31         31          
  Partials          0          0          

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[jmdobry]

@JustinBeckwith Looks good to you?