Improve handleStruct to use struct's own Unmarshaler if it is available

[michele]

handleStruct wasn't properly handling structs with custom marshaler/unmarshaler.

With this PR handleStruct correctly handles structs which implement the json.Unmarshaler interface.

This should fix #164

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[michele]

I signed it

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[mellena1]

Is there anything holding this back from getting merged? Worked for me with a custom struct holding a time.Time struct with a custom UnmarshalJSON method. I think this functionality is necessary for a lot of use cases.

[michele]

@mellena1 we've been using it for the past couple of months in production and it's working fine. Who's got the power to merge this? You can use my fork in the meanwhile.

[mellena1]

@michele I started using your fork last night and it’s working great for me so far. Thanks a lot for the work! Hopefully this can be merged in soon.

The only other thing I think would be good to add to this PR would be some documentation in the README, but it definitely works intuitively without it.

[alexfrancavilla]

@shwoodard Hi, could you take a look at this or dispatch it to somebody else with access to this project?