bug#63438: 30.0.50; abbrev mode wrongly capitalizes with mixed capitalized words

[Daniel Fleischer]

Recipe for recreating:

- Add some abbrev definition with mixed capitalization, e.g. "abc"
should expand to "aBC".
- Enable abbrev-mode. 
- Write "aBC".
- Upon pressing space, abbrev-mode converts it into "ABC". 

There is a code section in `abbrev-insert' that explicitly capitalizes
the first letter; I'm sure it's there for a reason but I think the
behavior I showed is unexpected. As for the example "aBC", there are
real use cases, just to mention one "iPhone".

-- 
Daniel Fleischer