fix: get value of register '=' with getreg('=',1)

[ccaseiro]

without the '1' the register gets evaluated and I get
'Vim(call):E474: Invalid argument'.

From vim documentation:
getreg('=') returns the last evaluated value of the expression
register. (For use in maps.)
getreg('=', 1) returns the expression itself, so that it can
be restored with |setreg()|. For other registers the extra
argument is ignored, thus you can always give it.

[folke]

Thanks!