Uppercase keys in FITSHeader

[jishnub]

FITS requires the header keywords be in uppercase. I wonder if it makes sense to convert the keys to uppercase while constructing a FITSHeader object? Otherwise the keys written out might differ from the ones read back in, which might cause unexpected bugs.

Alongside this, getindex on a FITSHeader might ignore the case of the key by converting it to uppercase. This will bring it closer to the behavior of astropy.

This might potentially be breaking though, but overall seems like a good change, as the FITSHeader object written out will be identical to the one read back in.