* Update ElixirClientCodegen.java
* Refine the regular expression for atoms
The original regex incorrectly matched `@atom` (unquoted atoms
cannot begin with @). However, through testing with `iex`, it also turns out that the atom
`:@` is legal.
The following atoms will now be quoted that would have been
incorrectly not quoted:
- `:@type`
* Add model to petstore client to test new atom creation regex pattern
