Add RequestParams#add

This is a shortcut for adding a parameter which can have many values; useful for communicating for something like Rails, where array parameters are non-unique.
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Merge pull request #275 from usepropeller/request_params_add

Add RequestParams#add
The failure message should have the response in a byte array.

handleSuccessMessage() was being (properly) handed a byte array, but handleFailureMessage() was being passed a string. This adjusts the call to handleFailureMessage() to pass a byte array as well.
Merge pull request #284 from fixedd/master

The failure message should have the response in a byte array.