HTTP Request\Response Header Grammar - http

Question

In the header of an HTTP request or response will the header keys be constant in terms of capitalization, between servers.
I ask so I can expect in my code: (Using Fake Function names)
Safe Precise Python Code
for hdr in header.keys():
if 'content-length' == hdr.lower():
recv_more_data( header[hdr] ) # header[hdr] == Content-Length (5388) bytes
break # Exit for loop when if statement is met.
Code I Would Like To Use
recv_more_data (header['Content-Length'])
# I know to expect 'Content-Length' not 'content-Length' or some other variation
Meaning will a server ever return a header with the keys like so.
Standard Request
GET / HTTP/1.1
Host: www.example-host.com
User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0
Accept: */*
Accept-Language: en-US
Accept-Encoding: gzip
Connection: closed
Content-Length: 0
A Bad But Possible Response?
HTTP/1.1 200 OK
Server: nginx/1.0.15
date: Thu, 23 Oct 2014 00:25:37 GMT
content-Type: text/html; charset=iso-8859-1
transfer-encoding: chunked
Connection: close
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Content-Encoding: gzip
Clarification will help my code neatness.

Answer 1

HTTP header names are case-insensitive, per the HTTP specification.
RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1
Section 4.2 - Message Headers
HTTP header fields, which include general-header (section 4.5),
request-header (section 5.3), response-header (section 6.2), and
entity-header (section 7.1) fields, follow the same generic format as
that given in Section 3.1 of RFC 822 [9]. Each header field consists
of a name followed by a colon (":") and the field value. Field names
are case-insensitive. The field value MAY be preceded by any amount
of LWS, though a single SP is preferred. Header fields can be
extended over multiple lines by preceding each extra line with at
least one SP or HT. Applications ought to follow "common form", where
one is known or indicated, when generating HTTP constructs, since
there might exist some implementations that fail to accept anything
beyond the common forms.
RFC 7230 - Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
Section 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.

Answer 2

HTTP header names are case insensitive.
It looks like you're using python. Check out the requests library. It'll make your life much easier: http://docs.python-requests.org/en/latest/

Answer 3

Bear in mind that, even though most major servers will have consistent capitalization, any Joe PHP Developer can set the response headers manually in their code - and there is no way to police what that guy uses as a capitalization standard.