Export 'camelTo' function and fix its tests

This function was added as part of #155, but never seems to have been exported. Looking at the tests for it, there seems to be a typo - the test uses "camlTo" - so it seems unlikely that it ever worked or even compiled. The test has been commented out in a more recent pull request, but indeed if I check out version 0.7.0.0 and run `cabal test`, I get

tests/Properties.hs:27:40: Not in scope: ‘camlTo’

This pull request exports the function from Data.Aeson.Types and fixes the tests.