Don't write to readonly property
Don't write to readonly property

This was caught at script load time and caused a spew of SpiderMonkey
strict mode warnings at the start of the test suite.

[endlessm/eos-sdk#3351]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib