inets: Add http_util:to_titlecase/1 to titleize header names

httpc: Use http_util:to_titlecase/1 to update header names

Although, header names are case-insensitive, but most commonly used
form of header names is titlecase among HTTP clients.
This commit would provide this behaviour, in which httpc would change
header names to titlecase before sending the request.
Merge pull request #6 from scrapinghub/send_header_names_in_titlecase

Send header names in titlecase with httpc