Add tests for mimedata

[blink1073]

--------------------|----------|----------|----------|----------|----------------|
File                |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
--------------------|----------|----------|----------|----------|----------------|
  mimedata.js       |      100 |      100 |      100 |      100 |

[coveralls]

Coverage increased (+0.01%) to 97.948% when pulling f1be49d on blink1073:mimedata-tests into 95c00bb on phosphorjs:master.

[sccolbert]

need a test which guarantees types() returns keys in order of calling setData (most recent last).

e.g.

setData('foo', 1)
setData('bar', 2)
setData('baz', 3)
setData('bar', 4)
clearData('foo')
setData('foo', 9)
types() === ['baz', 'bar', 'foo']

[sccolbert]

lgtm modulo my comment

[coveralls]

Coverage increased (+0.01%) to 97.948% when pulling 0c08f60 on blink1073:mimedata-tests into 95c00bb on phosphorjs:master.