It shouldn't matter whether the request body comes from a file or a
string, or whether it is passed to the Request constructor or assigned
later. It should always be stored internally as bytes.
When converting objects to body, dicts and sets order can change
resulting in a different but same body. This fixes the issue by
comparing the enclosed data in the body rather than the body itself
while still allowing raw body matching with the raw_body matcher.
This commit not only changes the default method of matching requests
(just match on method and URI instead of the entire request + headers)
but also allows the user to add custom matchers.
