THRIFT-5383 Fix misplaced capacity check

[aaronstgeorge-wf]

The added test passes with thrift 0.13.0 and fails on 0.14.0. The source of the error seems to be an unnecessary capacity check when reading JSON string.

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[aaronstgeorge-wf]

I'm not sure how we could know the size before reading the string? Because this is in the JSON protocol there is no preceding size packed in the data. All you have is the open quote character and you have to read to the end. This check makes sense for compact, binary, et al because they are serialized with the size of the string contained after, but the serialized JSON representation has no such thing. It's just a quote-delimited string.

[aaronstgeorge-wf]

I think we we got off on the wrong foot here. Where should that line be? I'm quite happy to move it and write a test that asserts it does what it is supposed to do. From what I know about the JSON protocol, I'm just not sure what it's supposed to do, or where it's supposed to be. I am not a thrift expert.