http: make adding a blank header thread-safe

http: make adding a blank header thread-safe

Previously the function would edit the provided header in-place when a
semicolon is used to signify an empty header. This made it impossible to
use the same set of custom headers in multiple threads simultaneously.

This approach now makes a local copy when it needs to edit the string.

Reported-by: d912e3 on github
Fixes #3578
Closes #3579