NMEA checksum should accept lowercase

[wellenvogel]

It seams that some manufacturers are sending the NMEA checksum using lower case characters for a...f.
So It would be good of the library would also accept them in the checksum.

[orca-hydromancer]

orca-io@706cd2e#diff-f880fa56145617b3efb131530a19dc1d2597d8065cc5b6281f8860cbb2925aaaR108

Never got around to creating a PR

[ttlappalainen]

This has been fixed already on last summer, but for some reason I had not pushed it to github. Now it is there wit some other updates I had on my own code.