This commit changes the routing algorithm. In particular, it enforces
precise matching of formats. With this change, a route with a specified
format only matches requests that have the same format specified. A
route with no format specified matches any request's format. This is
contrast to the previous behavior, where a route without a specified
format would match requests regardless of their format or whether one
was specified.
This commit also changes the following:
* The return type of the 'content_type' method of 'Request' is now
'Option<ContentType>'.
* The 'ContentType' request guard forwards when the request has no
specified ContentType.
* The 'add_header' and 'replace_header' methods take the header
argument generically.
Closes#120.
Sergio Benitez