Merge branch 'header_add_override' of https://github.com/kennypeng/nghttp2 into kennypeng-header_add_override

Library code has not changed at all since the last release, so LT
revision stays the same.

This is simply programming error, but it is interesting that using
libstdc++ does not reveal this error.  With clang++-libc++, we got
std::system_error: mutex lock faild: Invalid argument.  This is
because we did not give a name to lock object, so it is immediately
destructed.  I think this will fix the reported crash on Mac OSX.