Buffer.prototype.write does not properly truncate at character boundaries

[natevw]

A call to buf.write() should not write partial characters, whether limited by the target buffer length or explicit length parameter.

Code (explicit length case):

var b = Buffer(256);
b.fill(0);
var len = b.write("\u1234", 0, 2);
console.log(len, b.slice(0,3));

Expected:

0 <Buffer 00 00 00>

Actual:

2 <Buffer e1 88 00>

(There is a partial fix for the UTF-16 case in #692)