More tests!
The MediaTypeImmutable struct no longer contains a
pointer to a mutable struct. Containing a pointer was unnecessary and
dangerous, as it allowed the contained element to be modified outside
if not careful... breaking the immutability promise
Updating the `README.md` to show the new mutable
and immutable examples
Updating comments based on the `golint` rules
Renaming the mutable and immutable structs based on
the `golint` recommendation:

- MediaTypeMutable -> Mutable
- MediaTypeImmutable -> Immutable