Features:
- config param "key_claim_name" (default = "key"), so for example one could use "iss" to check the validity of the JWT;

Style:
- 2 blank lines between functions;
- 1 blank like before "else" and "elseif";
- jwt -> JWT;
- Capitalized logs and response messages;
- Added description for each schema configuration parameter;

I added a new test case for feature apache#11276

Since the default value of the new config parameter "key_claim_name" is "key", "default behaviour" is already validated by other tests.

+ Removed external httpbin upstream in favor of local endpoint (/hello);
+ Cleaned code;