sign command only checks build_path parameter's presence, not value (Version: 1.5 beta 5)

[carlhua]

Issue Description

When processing parameters, the sign command considers "build_path" to be enabled if the parameter is present in the JSON, regardless of what value it has. This is poor API design – because passing "build_path": false basically has the same behavior as "build_path": true.

rippled should actually check the boolean value of the parameter rather than just looking to see whether it's there.

Source:
https://github.com/ripple/rippled/blob/develop/src/ripple/rpc/impl/TransactionSign.cpp#L177-L185

Original reporter @mDuo13 - RIPD-1055

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Expected Result

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