Change datalog CSV download mime type from text/plain to text/csv

[martinwork]

Relates to #327.

[microbit-matt-hillsdon]

As things stand I think this PR will have no effect. See this doc for how to convert header.html into a change to the C++ file that embeds it. I can do this if needed.

I'm a bit puzzled as to why I didn't do this in the first place so I'd like to re-run some testing to make sure it doesn't make anything worse. It saves me two bytes in the header HTML which were very hard to come by! I'll try to do that this week.

[martinwork]

Thanks @microbit-matt-hillsdon

[microbit-carlos]

I've pushed a commit updating the MicroBitLog.cpp file and it's tripled the available free space from 1 byte to 3 🎉 🤣
@microbit-matt-hillsdon let us know once you've had a chance to run the tests you had in mind and we can merge the PR 👍

[JohnVidler]

Looks good to me, lets get this merged in. 👍

[microbit-carlos]

@microbit-matt-hillsdon / @microbit-robert this PR has been merged to CODAL, it mostly sets the CSV mime type from text/plain to text/csv. Letting you know as I assume the same mime type should probably be applied to any other CSV downloads for iOS.