Check case sensitivity uses

[jskeet]

We're fixing the use of CloudEvent attributes themselves not previously being case-sensitive, but every transport has its own rules, and we should check we're doing the right thing. In particular, often we're using an "invariant culture case-insensitive match" instead of an "ordinal case-insensitive match" and I don't know that that will always be the right approach.

[jskeet]

Places using "IgnoreCase":

• MimeUtilities (correct as per RFC 2045: "All media type values, subtype values, and parameter names as defined are case-insensitive"; unclear whether OrdinalIgnoreCase or InvariantCultureIgnoreCase is best, but it's very unlikely to matter)
• HttpClientExtensions/HttpListenerExtensions: web hook matching the "options" method; incorrect, as HTTP methods should be case-sensitive
• HttpUtilities prefix check (correct as headers are case-insensitive)
• KafkaClientExtensions: Unclear... need to find a Kafka expert

Of course, it's possible that there are other places that should be case-insensitive but aren't...

[jskeet]

Judging by RecordHeaders.java and Headers.cs, header keys are expected to be case-sensitive. Will fix.