[#621] Move extras_validation into __before

This fixes an issue where creating a new resource would crash in
validation if there was a custom field (using convert_to/from_extras)
with a name that sorted after extras validation alphabetically. Fixes #621.
[#621] Fix crash on duplicate extras key error

Fix a crash when generating a 'duplicate extras key' error. To trigger
the crash, post a dataset dict to package_create() containing two extras
dicts with the same key. There is no test for this yet.
[#621] Add tests for duplicate extras key error

This code path was not covered by the tests.
Merge branch 'master' of github.com:okfn/ckan into 621-bug-move-extras_validation_into___before
[#621] Return duplicate extras key error in a list

This makes it consistent with other errors that CKAN returns.
Merge branch 'master' into 621-bug-move-extras_validation_into___before
Merge branch '621-bug-move-extras_validation_into___before'