compression performance drop in v1.9 and v2.0

[flamesea12]

Hi

I recently found that compression performance drops down after I update to v1.9

I dug a little bit and found that there are many forward calls with a length of 
zero

Breakpoint 1, comp_http_forward_data (s=0x86a620, filter=0x873c70, 
msg=0x86b960, len=0) at src/flt_http_comp.c:386


which did not happen in version 1.8