Fix parsing of table size updates

[314eter]

When a table size update was not followed by a header, it resulted in a parsing failure. I fixed it by letting Decoder.header return a Header.t option, which is None if there are only table size updates in the input.

This situation can occur in:

• A header block that contains only table size updates without headers
  Empty header lists are allowed in theory, although they are probably never used.
• A non-empty header block that ends with a table size update
  It's unclear from the rfc whether table size updates in other places than the beginning of a header block are allowed, but most implementations do not support it and h2spec rejects it.