Missing `!` in header-mutating functions

[anowacki]

The following functions mutate their input, but do not have a trailing ! in the function name:

• setheader
• appendheader
• removeheader

This is unexpected; I think most people would have expected functions with these names to return a copy and leave the input unaltered. I would suggest treating this as a bug and renaming these functions setheader!, appendheader! and removeheader!, in line with functions like cancel!, setcookies!, etc.

[hhaensel]

I agree, I was also mislead by the naming

[quinnj]

Yeah........this was more of a continuing of previous naming, but probably right that we should be cleaner and at least more consistent here. I'm not sure I really want to deprecate the non-mutating, but maybe we add the mutating ! methods.

[anowacki]

Sorry, I didn't realise these functions also existed with these names in v1. I would agree that removing them now is probably too late.