Forbid JSON::ResumableParser#<< while the parser is in use

[mame]

Currently #<< is allowed even while in_use == true.

require "json"
parser = nil
parser = JSON::ResumableParser.new(on_load: ->(o) { parser << "2]" if o == 1; o })
parser << "[1,"
parser.parse   # => true
parser.value   # => [1, 2]   (the callback fed "2]" from inside the running parse)

Given that #clear is forbidden while in_use == true, this is probably not intentional? It may be harmless, but I think it is safer to forbid this code unless there is a clear use case.