adopt to compare string terminology.

[sassembla]

some server returns handshake headers with case-insensitive rule.

according to HTTP1.1(https://tools.ietf.org/html/rfc2616#section-2.1),
,

"literal"
Quotation marks surround literal text. Unless stated otherwise,
the text is case-insensitive.

also in RFC of WS(https://tools.ietf.org/html/rfc6455#section-4.1),

Comparing two strings in an ASCII case-insensitive manner means
comparing them exactly, code point for code point, except that the
characters in the range U+0041 to U+005A (i.e., LATIN CAPITAL LETTER
A to LATIN CAPITAL LETTER Z) and the corresponding characters in the
range U+0061 to U+007A (i.e., LATIN SMALL LETTER A to LATIN SMALL
LETTER Z) are considered to also match.

[eslindsey]

Is the compiler always cognizant of the fact that ("Sec-WebSocket-Extensions").ToLower() is constant, regardless of locale and optimization settings? If not, then this should be hard-coded as "sec-websocket-extensions" etc.

Perhaps a better way to implement the change you describe would be to use the mechanisms already built in to the NameValueCollection. In fact, from the documentation:

The comparer determines whether two keys are equal. The default comparer is the CaseInsensitiveComparer.

Did you run into a problem with the comparison being case-sensitive? From a brief look at the code, it seems to me it would be case-insensitive.