Support for case-insensitive headers

[Infocatcher]

Reported here: https://forum.mozilla-russia.org/viewtopic.php?pid=742599#p742599
Example: https://dl.google.com/dl/softwareremovaltool/win/software_removal_tool.exe
Response:

accept-ranges: bytes
content-length: 3923576
content-type: application/octet-stream
etag: "16a791"
last-modified: Wed, 30 Aug 2017 14:26:57 GMT
server: downloads
vary: *
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-xss-protection: 1; mode=block
date: Fri, 08 Sep 2017 08:24:46 GMT
alt-svc: quic=":443"; ma=2592000; v="39,38,37,35"
X-Firefox-Spdy: h2
X-NoScript-ReqData: {}

RFC 7230 - Hypertext Transfer Protocol (HTTP/1.1): 3.2. Header Fields

Each header field consists of a case-insensitive field name followed
by a colon (":"), optional leading whitespace, the field value, and
optional trailing whitespace.