Don't call to_json on string props

[justin808]

This bit of debugging shows the issue.

[1] (pry) #<#<Class:0x007f84e5bd19f8>>: 0> puts hash_or_string.to_json
"{\"comments\":[{\"id\":9,\"author\":\"xxx\",\"text\":\"xxxxxx\",\"created_at\":\"2017-01-11T08:14:10.296Z\",

[2] (pry) #<#<Class:0x007f84e5bd19f8>>: 0> puts hash_or_string
{"comments":[{"id":9,"author":"xxx","text":"xxxxxx","created_at":"2017-01-11T08:14:10.296Z",

We'll get the proper tests in #787.

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This change is 

[robwise]

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Reviewed 1 of 1 files at r1.
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Comments from Reviewable

[coveralls]

Coverage increased (+0.003%) to 97.922% when pulling 17a3858 on dont-call-to-json-on-props-converted-to-string-already into 7f04afd on master.