Enable to configure cljs-http.core/jsonp if response keys are keywordized

[jindrichmynarz]

I need cljs-http.core/jsonp not to keywordize response keys (https://github.com/r0man/cljs-http/blob/master/src/cljs_http/core.cljs#L116), because the keys in the JSON I'm requesting contain slashes and are thus (incorrectly) casted to namespaced keywords. Currently, keywordization is hard-coded, so it's not possible to disable it. Is it possible to add an option to configure if keys would be keywordized (defaulting to true to maintain compatibility)?

[r0man]

@jindrichmynarz Yes, patch with test welcome!

[jindrichmynarz]

I drafted a solution to this issue in #87, aiming for minimal changes to the library. Additional changes may include reducing the boilerplate in JSON-P tests, reusing cljs-http.util/json-decode in cljs-http.core/jsonp, and enabling to set :keywordize-keys there as well.