Make headers case-insensitive

[Doddlin]

Describe the bug
HTTP Spec linked below suggests that headers should be case-insensitive

To Reproduce
Using an NodeMCU running ESP8266 as a server and an iOS Simulator to connect (using dart) declines the connection since the header sent is lower case.

Expected behavior
I would expect to be able to connect no matter what case my headers are sent as.

Additional context
Link to HTTP Spec.
https://tools.ietf.org/html/rfc2616#section-4.2

[gilmaimon]

Really good and important suggestion, I'm opening this for PR's :)

[khoih-prog]

Dear @gilmaimon

I already add this feature to the WebSockets2_Generic library, which is based on this library of yours.

I'll make a PR for this library shortly.

Best Regards,

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Major Release v1.10.0

1. Supporting case-insensitive headers, according to RFC2616
2. Sync with ArduinoWebsockets releases v0.5.3

• Add connectSecure method to support WSS connection with the classic interface (host, port, path)
• Add 1 second connection timeout for ESP32/ESP8266, Teensy 4.1 NativeEthernet and QNEthernet

3. Improve debug

[khoih-prog]

PR has been created as Supporting case-insensitive headers #128