autoprefixer: Doesn't properly generate input placeholder rules

source:

input::placeholder { color: black; }
input::-webkit-input-placeholder { line-height: 16px; /* fix chrome bug */ }

output:

input::-moz-placeholder { color: black; }
input:-ms-input-placeholder { color: black; }
input::placeholder { color: black; }
input::-webkit-input-placeholder { line-height: 16px; /* fix chrome bug */ }

expected:

input::-moz-placeholder { color: black; }
input:-ms-input-placeholder { color: black; }
input::-webkit-input-placeholder { color: black; }
input::placeholder { color: black; }
input::-webkit-input-placeholder { line-height: 16px; /* fix chrome bug */ }

I’d expect to be able to “extend” the generated webkit rule (in the example), rather than have it completely omitted by AP.