shrpx: Made SPDY/3 default protocol in SPDY proxy mode
shrpx: Made SPDY/3 default protocol in SPDY proxy mode

The reason why we choose SPDY/2 as default for SPDY prxy was due to
Chrome's window update bug. Now its fix is available in Chrome stable,
we make SPDY/3 as default.

dxq-git/nghttp2,bxshi/nghttp2,ahnan4arch/spdylay,tatsuhiro-t/spdylay,ahnan4arch/spdylay,mixianghang/nghttp2,shines77/nghttp2,kelbyludwig/nghttp2,mixianghang/spdylay,syohex/nghttp2,tatsuhiro-t/spdylay,thinred/nghttp2,serioussam/nghttp2,mixianghang/spdylay,ohyeah521/nghttp2,shines77/nghttp2,yuki-kodama/nghttp2,dxq-git/nghttp2,ahnan4arch/spdylay,icing/nghttp2,ahnan4arch/spdylay,mixianghang/nghttp2,minhoryang/nghttp2,bxshi/nghttp2,thinred/nghttp2,lukw00/nghttp2,lukw00/nghttp2,wzyboy/nghttp2,serioussam/nghttp2,icing/nghttp2,minhoryang/nghttp2,dxq-git/nghttp2,yuki-kodama/nghttp2,syohex/nghttp2,thinred/nghttp2,lukw00/nghttp2,tatsuhiro-t/spdylay,syohex/nghttp2,wzyboy/nghttp2,thinred/nghttp2,dxq-git/nghttp2,wzyboy/nghttp2,serioussam/nghttp2,mixianghang/spdylay,shines77/nghttp2,bxshi/nghttp2,minhoryang/nghttp2,minhoryang/nghttp2,wzyboy/nghttp2,bxshi/nghttp2,shines77/nghttp2,mixianghang/nghttp2,syohex/nghttp2,dxq-git/nghttp2,lukw00/nghttp2,mixianghang/nghttp2,lukw00/nghttp2,thinred/nghttp2,icing/nghttp2,kelbyludwig/nghttp2,icing/nghttp2,ohyeah521/nghttp2,ohyeah521/nghttp2,minhoryang/nghttp2,icing/nghttp2,wzyboy/nghttp2,ohyeah521/nghttp2,yuki-kodama/nghttp2,mixianghang/spdylay,ahnan4arch/spdylay,lukw00/nghttp2,serioussam/nghttp2,lukw00/nghttp2,kelbyludwig/nghttp2,yuki-kodama/nghttp2,thinred/nghttp2,yuki-kodama/nghttp2,bxshi/nghttp2,ohyeah521/nghttp2,serioussam/nghttp2,kelbyludwig/nghttp2,serioussam/nghttp2,dxq-git/nghttp2,shines77/nghttp2,ohyeah521/nghttp2,icing/nghttp2,mixianghang/nghttp2,mixianghang/spdylay,syohex/nghttp2,kelbyludwig/nghttp2,bxshi/nghttp2,icing/nghttp2,tatsuhiro-t/spdylay,syohex/nghttp2,minhoryang/nghttp2,serioussam/nghttp2,wzyboy/nghttp2,yuki-kodama/nghttp2,mixianghang/nghttp2,tatsuhiro-t/spdylay,shines77/nghttp2,wzyboy/nghttp2,kelbyludwig/nghttp2,thinred/nghttp2