Fixes the TypeScript definitions for AxiosHeaders:

1. Adds Content-Encoding header as an accessor. The {get,set,has}ContentEncoding methods declared in the types and documented in the README did not actually exist at runtime and would throw an error. Now the runtime behavior matches the types.
2. Adds missing types for the {get,set,has}AcceptEncoding methods created by the Accept-Encoding accessor
3. Fixes return type of the no-arg overload for get accessors. For example, headers.getContentType() returns the value of the content-type header. The return type was incorrectly declared as RegExpExecArray | null when not passing a RegExp.