Update _pxToEm.scss

[MrFjords]

Added a check for if the value 'auto' is passed to the function to deal with cases where auto is desired as an output. for example:

margin: pxToEm(20px auto, 16px); would output -> margin: 1.25em auto;

[stubbornella]

@mobywhale-chan can you code review this PR?

[mobywhale-chan]

@MrFjords Looks good. Thanks!

[drublic]

You might also want to use percentage based or other values.
I personally use something similar to

@if $value == 0 or type_of($value) != number

in my px to rem mixin over here.

It might be interesting to use a mixin like that for rem too and combine it with the fontSizes mixin.

[MrFjords]

If I've needed to use percentages along with ems I tend to go with something like this:

margin: pxToEm(20px, 16px) percentage(20px/$widthOfContent);