Allow request caching to be overridden

[andyjpeck]

Allow request caching to be overridden by the user. This makes it possible to disable caching when making 'GET' requests.

This resolves: https://github.com/google/streamy-dart/issues/75

[yjbanov]

This makes the Request class aware of caching, which we consider an orthogonal concept. There are existing alternatives to what you are trying to achieve:

• To override cache behavior at call site, define a custom bool sendParam in your streamy.yaml file, e.g. cacheable. This will add a "cacheable" key to request.local map. Then create a custom cache implementation that doesn't cache requests with request.local["cacheable"] == false. Then the call sites can do:

api.foos.get(123).send(cacheable: false).listen(...);

• To override cache behavior based on request type (or any other statically defined rule), simply use a custom cache implementation.