In http_EstimateWS(), all headers are passed to the http_isfiltered()
function to calculate how many bytes is needed to serialize the entire
struct http. http_isfiltered() will check the headers for whether they are
going to be filtered out later and if so skip them.

However http_isfiltered() would attempt to treat all elements of struct
http as regular headers with an implicit structure. That does not hold for
the first three pseudo-header entries, which would lead to asserts in
later steps.

This patch skips the filter step for pseudo-headers.

Fixes: #3830