the maxBuffer isn't complete accurate in child_process

[JacksonTian]

The maxBuffer is not complete accurate when data include unicode and encoding is set.

[Trott]

#1902 would (if landed) be a complete fix for this, right?

[JacksonTian]

@Trott the #1902 depends on bfb2cd0 , but it was reverted because it not correct in all cases.