Serialization function should be applied on ChoiceType output

[philiptzou]

This fixed #9.

[coveralls]

Coverage decreased (-1.5%) to 88.755% when pulling 0760abe on philiptzou:patch-1 into eb4df8d on graphql-python:master.

[coveralls]

Coverage decreased (-1.5%) to 88.755% when pulling f834dda on philiptzou:patch-1 into eb4df8d on graphql-python:master.

[coveralls]

Coverage increased (+0.1%) to 90.361% when pulling 5940556 on philiptzou:patch-1 into eb4df8d on graphql-python:master.

[syrusakbary]

Sorry for addressing this so late.

This PR (and related PRs mentioned in #9) didn't got merged because they were based on making enums serialization "dynamic" (meaning modifiable if indicated by the enum configuration) and that might eventually cause some inconsistency as Enum is a fixed concept and it's serialization and parsing should be uniform across any project.

However I think this issue might be easily solved without the need of modifying it's serialization.

Closing this PR in favor of another PR with a test case that exemplifies this issue (without depending on GraphQLEnum.serialize) so we make sure can be addressed without the need of modifying graphql-core and graphene.