OTA MD5 hash check should be case-insensitive

[vshymanskyy]

I faced this kind of log, which is obviously not how it should perform:

MD5 Failed: expected:72CF9F05E23E2DF2080E376A661E04CB, calculated:72cf9f05e23e2df2080e376a661e04cb

ESP8266httpUpdate, The latest stable version, 2.4.2.