Investigate client reports of invalid dh

[jrconlin]

See: mozilla/application-services#3364

[rfk]

TL;DR the rust code doesn't handle quoted values when parsing the Encryption or Crypto-Key headers, ref mozilla/application-services#3365

I think it's some to close this, unless you think it'd be worth autopush doing a bit of normalization here on the server side.

[jrconlin]

It's not a bad idea for the server to normalize values where appropriate. The opposite option would be to reject the request as invalid, but I think that might harm the library consumers more than the library authors.

It's a minor fix in any case, so we can at least consider it on the server. I still think the client should also normalize where and when possible in order to avoid future fragility issues.