optionalField produces wrong error on non-object JSON

[mlms13]

jsonString
|> optionalField("field", string)
|> Result.mapError(ParseError.failureToDebugString)

I would expect the above expression to produce an error to the effect of: "Expected object but found \"string\"". Instead:

Failed to decode object:
    Field \"field\" had an invalid value: Expected object but found \"string\"

This is wrong. The field "field" didn't have an invalid value... it wasn't there at all because the whole thing is not an object.

[mlms13]

I found this while consolidating tests in the v2 branch, but I'll include the fix in 1.2.