[I2C] bus and speed are not a readonly properties

[haoxli]

dictionary I2CInit {
    octet bus;
    I2CBusSpeed speed;
};

interface I2CBus {
    // has all the properties of I2CInit as read-only attributes
};

bus and speed should be readonly properties, but actually it can be modified.

var i2cDevice = i2c.open({ bus: 0, speed: 100 });
var busValue = i2cDevice.bus;
var speedValue = i2cDevice.speed;
i2cDevice.bus = busValue + 1;
i2cDevice.speed = speedValue + 1;
console.log(i2cDevice.bus);
console.log(i2cDevice.speed);

The expected result are 0 and 100, but print 1 and 101.

Branch	Build ID	Date	Result
master	commit 2a9bbc7	Nov 25, 2016	Fail
zjs-0.1	commit dcb06fb	Nov 25, 2016	Fail

[grgustaf]

Fixed by #576. Improves pass rate of test-i2c.js to 12/12.

[xiaolongx]

This issue cannot reproduce on both Arduino 101 and FRDM-K64F with commit 2345f96