`socket.headers.canonic`: use dynamic canonization instead of limited set of predefined values

[johnd0e]

Fix #440

[johnd0e]

Note: I would rather use "%f[^-]%l", but it doesn't match the beginning of the string.

To be even perfecter we could include other theoretically possible chars besides %w, but the RFC is not clear for me here https://www.rfc-editor.org/rfc/rfc7230#section-3.2