`set` command values parsed inaccurately

Note that in the following **there is a blank after the backslash**
```vim
set lbr=+++\ 
```
generates
```
pcall(vim.cmd, [[ set lbr=+++ ]])
```
The trailing blank is lost. Perhaps this can be seen more clearly in the following where the backslash is converted to a space.
```
set tags=foo\ bar
```
generates
```
pcall(vim.cmd, [[ set tags=foo  bar ]])
```
