Honor as_json and reject disabled strict mode in JSON::Coder

[ydah]

Coder overwrites the as_json option with nil when no block is given. It also silently overrides strict: false.

require "json"

JSON::Coder.new(as_json: ->(object, is_key) { 42 }).dump(Object.new)
# Before: JSON::GeneratorError
# After:  "42"

JSON::Coder.new(strict: false)
# Before: silently enables strict mode
# After:  ArgumentError

Restore the keyword callback while preserving block precedence. Keep Coder's documented strict-mode requirement and reject attempts to disable it.