issue #798: Lower case http headers from arango

This change allows returning capitalized HTTP headers, e.g.
`Content-Length` instead of `content-length`.
The HTTP spec says that headers are case-insensitive, but
in fact several clients rely on a specific case in response
headers.
This change will capitalize HTTP headers if the `X-Arango-Version`
request header is sent by the client and contains a value of at
least `20100` (for version 2.1). The default value for the
compatibility can also be set at server start, using the
`--server.default-api-compatibility` option.
updated CHANGELOG
Merge branch 'devel' of https://github.com/triAGENS/ArangoDB into devel