Field name aliasing broken if naming strategy used

[shdunning]

One of the cool features of this library is that we can set field naming strategy like so...

extra.SetNamingStrategy(extra.LowerCaseWithUnderscores)

..however, then we can no longer rename fields using json tags; that is, the naming strategy forces the responses to always follow the naming strategy. This can be too restrictive in a production environment where we want a separation between the data and API models.

PR #275 proposes a fix for this issue.