This changes the renderer class to store the controller and defaults as
an instance variable rather than allocating a new class. You can create
a new renderer with an new env by calling `Renderer#new` or use new
defaults by calling `Renderer#with_defaults` and saving the return value
somewhere.
Also I want to keep the `env` private since I would like to change the
keys in the future. This commit only translates particular keys that
the user requested.
Previously env was duplicated and then had it's keys mutated. This iterates through
the hash twice.
Using `transform_keys`, duplication and key mutation is a single iteration.
`convert_symbols` was renamed to `http_header_format`.
