We attempt to do a special encoding trick when serializing
`additional_owners` under an account: when updating a value, we actually
send the update parameters up as an integer-indexed hash rather than an
array. So instead of this:
field[]=item1&field[]=item2&field[]=item3
We send this:
field[0]=item1&field[1]=item2&field[2]=item3
The trouble is that this had previously been built into the core library
as the default handling for all arrays. Because of this, it was
impossible to resize a non-`additional_owners` array as described in
more detail in #340.
This patch special cases `additional_owners` and brings sane behavior
back to normal arrays along with a test suite so that we try to build
some better guarantees around both the general and non-general cases.