Rename http header fields since can be both in request/response

[awick]

Can't search fields that can be in both request/response. For example adding content-type to both request and response headers creates a single http.content-type expression and which it actually searches is unknown. Probably should create http.request.content-type and http.response.content-type or something.

Work around for now is

[custom-fields]
http.request.content-type=db:http.request-content-type;kind:termfield;count:true
http.response.content-type=db:http.response-content-type;kind:termfield;count:true

Will need to fix the http.detail.jade file up too

[awick]

fixed in #2171