Standardize Content-Type capitalization

While apparently the header fields are supposed to be case insensitive,
 #142 shows at least some systems do not follow the spec properly.  So
we might as well standardize all uses of `Content-Type` in the same
way.
Add note to news about Content-Type
Fix broken merge

There was an extra block I missed when I was doing the merge
Merge pull request #146 from jimhester/Content-Type

Standardize Content-Type capitalization