We forbid empty keys or keys with '=' because it leads to ambiguous parsing.
Relevent PAX specification:
<<<
A keyword shall not include an <equals-sign>.
>
> Also, we forbid the writer from encoding records with an empty value.
> While, this is a valid record syntactically, the semantics of an empty
> value is that previous records with that key should be deleted.
> Since we have no support (and probably never will) for global PAX records,
> deletion is a non-sensible operation.
> <<<
> If the <value> field is zero length,
> it shall delete any header block field,
> previously entered extended header value,
> or global extended header value of the same name.

Fixes #20698
Fixes #15567

Change-Id: Ia29c5c6ef2e36cd9e6d7f6cff10e92b96a62f0d1
Reviewed-on: https://go-review.googlesource.com/55571Reviewed-by: Ian Lance Taylor <iant@golang.org>