The current logic in writeHeader attempts to encode the Header in one
format and if it discovered that it could not it would attempt to
switch to a different format mid-way through. This makes it very
hard to reason about what format will be used in the end and whether
it will even be a valid format.

Instead, we should verify from the start what formats are allowed
to encode the given input Header. If no formats are possible,
then we can return immediately, rejecting the Header.

For now, we continue on to the hairy logic in writeHeader, but
a future CL can split that logic up and specialize them for each
format now that we know what is possible.

Update #9683
Update #12594

Change-Id: I8406ea855dfcb8b478a03a7058ddf8b2b09d46dc
Reviewed-on: https://go-review.googlesource.com/54433
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>