Reserve room for 16 response headers by default.

In my data, most often the final size of fetchResponse is 16.
By reserving that data upfront, we can get rid of 3/4 of the
list allocations, according to the growth-strategy outlined here:
http://qt-project.org/doc/qt-4.8/containers.html#growth-strategies
I.e. before, we'd allocate 4, 8, 12, 16. Now we directly allocate
room for 16.

REVIEW: 121408